This applies spec changes from ethereum/EIPs#1049 and adds support for
pluggable identity schemes.

Some care has been taken to make the "v4" scheme standalone. It uses
public APIs only and could be moved out of package enr at any time.

A couple of minor changes were needed to make identity schemes work:

- The sequence number is now updated in Set instead of when signing.
- Record is now copy-safe, i.e. calling Set on a shallow copy doesn't
  modify the record it was copied from.

travis: try to upgrade android builder to trusty

Go code users can now tag event struct members with `abi:` to specify in what fields the event will be de-serialized.

See PR #16648 for details.

* bmt/*: golint updates for this or self warning

* Update bmt.go

Feed keeps active subscription channels in a slice called 'f.sendCases'.
The Send method tracks the active cases in a local variable 'cases'
whose value is f.sendCases initially. 'cases' shrinks to a shorter
prefix of f.sendCases every time a send succeeds, moving the successful
case out of range of the active case list.

This can be confusing because the two slices share a backing array. Add
more comments to document what is going on. Also add a test for removing
a case that is in 'f.sentCases' but not 'cases'.

* all: get rid of error when create mdb

* core: clean up variables definition

* all: inline mdb definition

* eth/filter: check nil pointer when unsubscribe

* eth/filters, accounts, rpc: abort system if subscribe failed

* eth/filter: add crit log before exit

* eth/filter, event: minor fixes

* cmd: various golint fixes

* cmd: update to pr change request

* cmd: update to pr change request

Peer.run was discarding the reason for disconnection sent to the disc
channel by Disconnect.

* whisper/mailserver: pass init error to the caller

* whisper/mailserver: add returns to fmt.Errorf

* whisper/mailserver: check err in mailserver init test

* signer/*: golint fixes

Specifically naming and comment formatting for documentation

* signer/*: fixed naming error crashing build

* signer/*: corrected error

* signer/core: fix tiny error whitespace

* signer/rules: fix test refactor

* eth/*:golint updates for this or self warning

* eth/*: golint updates for this or self warning, pr updated per feedback

* event/*: golint updates for this or self warning

* event/*: golint updates for this or self warning, pr updated per feedback

* metrics/*: golint updates for this or self warning

* metrics/*: golint updates for this or self warning, updated pr from feedback

* rpc/*: golint error with context as last parameter

* Update json.go

clef: documentation about setup

build: Add ldflags "-s -w" when building aar