There is no need to depend on the old context package now that the
minimum Go version is 1.7. The move to "context" eliminates our weird
vendoring setup. Some vendored code still uses golang.org/x/net/context
and it is now vendored in the normal way.

This change triggered new vet checks around context.WithTimeout which
didn't fire with golang.org/x/net/context.

* les: implement request distributor, fix blocking issues
* core: moved header validation before chain mutex lock

build: require Go >= 1.7

We don't use the opencl build tag anymore.

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.

core/types: use gencodec for JSON marshaling code

travis: switch to cocoapods 1.2.0 stable

ethstats: try both ws:// and wss:// if none specified

Fixes #3762.  Details about parameter:
https://github.com/rs/cors/blob/a62a804a8a009876ca59105f7899938a1349f4b3/cors.go#L50-L54

light: added new CHT

* main,import: Add --nocompaction flag + multiple import files

* geth/import: documentation

* import: Added more info to err message

* fix :P

mobile: Fix typo ( Ethereun -> Ethereum )

all: update light logs (and a few others) to the new model

core: reorg logs crashed, add a check for corner cases

build: fix xgo argument order when building from make