build: pull in ci.go dependencies for the PPA builder

* go.mod, vendor: switch to Go modules

* travis: explicitly enable go modules in Go 1.11 and 1.12

* accounts/abi/bind: switch binding test to go modules

* travis, build: aggregate and upload go mod dependencies for PPA

* go.mod: tidy up the modules to avoid xgo writes to go.sum

* build, internal/build: drop own file/folder copier

* travis: fake build ppa only for go module dependencies

* mobile: fix CopyFile switch to package cp

* build, travis: use ephemeral debsrc GOPATH to get mod deps

cmd/puppeth: make ssh prompt more user-friendly

cmd/puppeth: x-spec nonce data type, use types.BlockNonce

Refactors to use existing BlockNonce type instead of
hand-rolled bytes logic.

tests: refactor TestState to dedupe walk callback

Minor refactoring.

internal/ethapi: don't query wallets at every execution of gas estimation

* miner: fix data race in tests

miner: fix linter

* miner: address comment

* rpc: remove 'exported or builtin' restriction for parameters

There is no technial reason for this restriction because package reflect
can create values of any type. Requiring parameters and return values to
be exported causes a lot of noise in package exports.

* rpc: fix staticcheck warnings

Also fix the input formatter on clique_getSnapshot and clique_getSigners
so that integers as well as hex number strings are accepted.

* core/rawdb: check hash before return data from ancient db

* core/rawdb: fix lint

* core/rawdb: calculate the hash in the fly

Found by staticcheck.

Also unexport the status struct.

This PR introduces clique_status which gives info about the health of
the clique network.

It's currently a bit PITA to find out how a clique network is
performing, and it can easily happen that sealers drop off -- and
everything is 'fine' until one more signer drops off, and the network
suddenly halts.

The new method provides the following stats:

- Which signers are currently active, and have signed blocks in the last
  N (set to 64) blocks?
- How many blocks has each signer signed?
- What is the difficulty in the last N blocks, compared to the
  theoretical maximum?