* travis: exclude non-test jobs for PRs

We don't usually look at these builders and not starting them
removes ~15min of build time.

* build: don't run vet before tests

Recent versions of Go run vet during 'go test' and we have
a dedicated lint job.

* build: use -timeout 5m for tests

Tests sometimes hang on Travis. CI runs are aborted after 10min with no
output. Adding the timeout means we get to see the stack trace for
timeouts.

build: rename swarm deb package to ethereum-swarm; change swarm deb version from 1.8.x to 0.3.x (#16988)

* build: add support for different package and binary names

* build: bump up copyright date

* build: change default PackageName to empty string

* build, internal, swarm: enhance build/release process

* build: hack ethereum-swarm as a "depends" in deb package

* build/ci: remove redundant variables

* build, cmd, mobile, params, swarm: remove VERSION file; rename Version to VersionMeta;

* internal: remove VERSION() method which reads VERSION file

* build: fix VersionFilePath to Version

* Makefile: remove clean_go_build_cache.sh until it works

* Makefile: revert removal of clean_go_build_cache.sh

This commit adds all changes needed for the merge of swarm-network-rewrite.
The changes:

- build: increase linter timeout
- contracts/ens: export ensNode
- log: add Output method and enable fractional seconds in format
- metrics: relax test timeout
- p2p: reduced some log levels, updates to simulation packages
- rpc: increased maxClientSubscriptionBuffer to 20000

Smaller size on mobile is always good.
Might also solve our maven central upload problem

* build: add GOBIN to PATH for gomobile

* build: install gobind alongside gomobile

* crypto/bn256: full switchover to cloudflare's code

* crypto/bn256: only use cloudflare for optimized architectures

* crypto/bn256: upstream fallback for non-optimized code

* .travis, build: drop support for Go 1.8 (need type aliases)

* crypto/bn256/cloudflare: enable curve mul lattice optimization

This avoids setting CC for the go run invocation, which fails on go1.10.

* build: enable unconvert linter

 - fixes #15453
 - update code base for failing cases

* cmd/puppeth: replace syscall.Stdin with os.Stdin.Fd() for unconvert linter

* build: [finishes #15372] implements generalized linter and travis job

* .travis, build: minor polishes, disable deadcode