This adds an implementation of node discovery via DNS TXT records to the
go-ethereum library. The implementation doesn't match EIP-1459 exactly,
the main difference being that this implementation uses separate merkle
trees for tree links and ENRs. The EIP will be updated to match p2p/dnsdisc.

To maintain DNS trees, cmd/devp2p provides a frontend for the p2p/dnsdisc
library. The new 'dns' subcommands can be used to create, sign and deploy DNS
discovery trees.

dashboard: log dashboard url

* core/state: revert noop finalise, fix copy-commit-copy

* core/state: reintroduce net sstore tracking, extend tests for it

params: remove legacy bootnodes

deepCopy didn't copy pending storage updates, leading to the
creation of blocks with invalid state root.

vendor: pull in USB Windows fixes

params: activate Istanbul on Ropsten, Rinkeby and Görli

params: bump CHTs for the 1.9.4 release

core: fix tx dedup return error count

* les: detailed relative cost metrics

* les: filter txpool relative request statistic

* les: initialize price factors

* les: increased connected bias to lower churn rate

* les: fixed clientPool.setLimits

* core: do not use mutex in GetAncestor

* les: bump factor db version again

* les: add metrics

* les, light: minor fixes

core: dedup known transactions without global lock, track metrics

* txpool: smaller lock portion

* core/tx_pool: fix data race

cmd/evm: make evm default to all ethash protocol changes

This change adds tests for the virtual clock and aligns the interface
with the time package by renaming Cancel to Stop. It also removes the
binary search from Stop because it complicates the code unnecessarily.

core/state: accumulate writes and only update tries when must

* rlp: improve nil pointer handling

In both encoder and decoder, the rules for encoding nil pointers were a
bit hard to understand, and didn't leave much choice. Since RLP allows
two empty values (empty list, empty string), any protocol built on RLP
must choose either of these values to represent the null value in a
certain context.

This change adds choice in the form of two new struct tags, "nilString"
and "nilList". These can be used to specify how a nil pointer value is
encoded. The "nil" tag still exists, but its implementation is now
explicit and defines exactly how nil pointers are handled in a single
place.

Another important change in this commit is how nil pointers and the
Encoder interface interact. The EncodeRLP method was previously called
even on nil values, which was supposed to give users a choice of how
their value would be handled when nil. It turns out this is a stupid
idea. If you create a network protocol containing an object defined in
another package, it's better to be able to say that the object should be
a list or string when nil in the definition of the protocol message
rather than defining the encoding of nil on the object itself.

As of this commit, the encoding rules for pointers now take precedence
over the Encoder interface rule. I think the "nil" tag will work fine
for most cases. For special kinds of objects which are a struct in Go
but strings in RLP, code using the object can specify the desired
encoding of nil using the "nilString" and "nilList" tags.

* rlp: propagate struct field type errors

If a struct contained fields of undecodable type, the encoder and
decoder would panic instead of returning an error. Fix this by
propagating type errors in makeStruct{Writer,Decoder} and add a test.

build: switch PPA from Gophers dep to manual download

travis, Dockerfile, appveyor: bump to Go 1.13

Most of these changes are related to the Go 1.13 changes to test binary
flag handling. 

* cmd/geth: make attach tests more reliable

This makes the test wait for the endpoint to come up by polling
it instead of waiting for two seconds.

* tests: fix test binary flags for Go 1.13

Calling flag.Parse during package initialization is prohibited
as of Go 1.13 and causes test failures. Call it in TestMain instead.

* crypto/ecies: remove useless -dump flag in tests

* p2p/simulations: fix test binary flags for Go 1.13

Calling flag.Parse during package initialization is prohibited
as of Go 1.13 and causes test failures. Call it in TestMain instead.

* build: remove workaround for ./... vendor matching

This workaround was necessary for Go 1.8. The Go 1.9 release changed
the expansion rules to exclude vendored packages.

* Makefile: use relative path for GOBIN

This makes the "Run ./build/bin/..." line look nicer.

* les: fix test binary flags for Go 1.13

Calling flag.Parse during package initialization is prohibited
as of Go 1.13 and causes test failures. Call it in TestMain instead.