* travis, appveyor: bump to Go 1.11.4

* internal/build: revert comment changes

* 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.

This commit is meant to allow ecosystem projects such as ethersphere
to minimize CI build times by specifying an environment variable with
the packages to run tests on.

If the environment variable isn't defined the build script will test
all packages so this shouldn't affect the main go-ethereum repository.

This is supposed to fix the FTP upload issue according to
travis-ci/travis-ci#9391.

* 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

* travis.yml: work around Go 1.9.4 issue

* travis: workaround the workaround

This works around travis-ci/travis-ci#8703.

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

Also remove installation steps for fuse and golang.org/x/tools/cmd/cover 
because they're not required anymore.

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

* .travis, build: minor polishes, disable deadcode

This commit adds a build step to travis to auto-delete unstable archives older than
14 days (our regular release schedule) from Azure via ci.go purge.

The commit also pulls in the latest Azure storage code, also switching over from
the old import path (github.com/Azure/azure-sdk-for-go) to the new split one
(github.com/Azure/azure-storage-go).

This ensures 'make test' finds all errors that remote CI would find.
Go 1.7 vet reports a false positive in package log, add a workaround.

We have decided to bump the requirement to Go 1.7 because it enables
subtests and allows dropping backwards-compatibility code. This is in
line with Go's support policy. Go 1.6 and earlier no longer receive
security updates.