* core: fix staticcheck warnings

* fix goimports

* p2p/discover: slow down lookups on empty table

* p2p/discover: wake from slowdown sleep when table is closed

* accounts/keystore: fix staticcheck warnings

* review feedback

* rlp: fix staticcheck warnings

* rlp: fix ExampleDecode test

cmd/geth: remove network id from version cmd

It was reflective only of the Default setting,
and not chain aware.

Numbers were already allowed when creating labels, just not when
referencing them.

* p2p/discv5: add deprecation warning and remove unused code

* p2p/discv5: remove unused variables

Also remove dependency on gopkg.in/check.v1 in tests.

* accounts/abi: fix staticcheck warnings

* accounts/abi: restore unused field for test

build: skip go clean on PPA, messes with the module trick

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.