* build: use golangci-lint
This changes build/ci.go to download and run golangci-lint instead
of gometalinter.
* core/state: fix unnecessary conversion
* p2p/simulations: fix lock copying (found by go vet)
* signer/core: fix unnecessary conversions
* crypto/ecies: remove unused function cmpPublic
* core/rawdb: remove unused function print
* core/state: remove unused function xTestFuzzCutter
* core/vm: disable TestWriteExpectedValues in a different way
* core/forkid: remove unused function checksum
* les: remove unused type proofsData
* cmd/utils: remove unused functions prefixedNames, prefixFor
* crypto/bn256: run goimports
* p2p/nat: fix goimports lint issue
* cmd/clef: avoid using unkeyed struct fields
* les: cancel context in testRequest
* rlp: delete unreachable code
* core: gofmt
* internal/build: simplify DownloadFile for Go 1.11 compatibility
* build: remove go test --short flag
* .travis.yml: disable build cache
* whisper/whisperv6: fix ineffectual assignment in TestWhisperIdentityManagement
* .golangci.yml: enable goconst and ineffassign linters
* build: print message when there are no lint issues
* internal/build: refactor download a bit
* build: bump PPAs to Go 1.13 (via longsleep), keep Trusty on 1.11
* travis, build, vendor: use own Go bundle for PPA builds
* travis, build, internal, vendor: smarter Go bundler, own untar
* build: updated ci-notes with new Go bundling, only make, don't test
The new build script, ci.go, replaces some of the older shell scripts.
ci.go can compile go-ethereum, run the tests, create release archives
and debian source packages.
