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.

event: address Feed review issues

event: clarify role of NewSubscription function

event: more Feed review fixes

* take sendLock after dropping f.mu
* add constant for number of special cases

event: fix subscribing/unsubscribing while Send is blocked

This commit introduces a new Subscription type, which is synonymous with
ethereum.Subscription. It also adds a couple of utilities that make
working with Subscriptions easier. The mot complex utility is Feed, a
synchronisation device that implements broadcast subscriptions. Feed is
slightly faster than TypeMux and will replace uses of TypeMux across the
go-ethereum codebase in the future.

The Subscription type is gone, all uses are replaced by
*TypeMuxSubscription. This change is prep-work for the
introduction of the new Subscription type in a later commit.

   gorename -from '"github.com/ethereum/go-ethereum/event"::Event' -to TypeMuxEvent
   gorename -from '"github.com/ethereum/go-ethereum/event"::muxsub' -to TypeMuxSubscription
   gofmt -w -r 'Subscription -> *TypeMuxSubscription' ./event/*.go
   find . -name '*.go' -and -not -regex '\./vendor/.*' \| xargs gofmt -w -r 'event.Subscription -> *event.TypeMuxSubscription'

This fixes an issue where the following would lead to a panic due to a
channel being closed twice:

* Start mux
* Stop mux
* Sub to mux
* Unsub

This is fixed by setting the subscriptions status to closed resulting in
the Unsubscribe to ignore the request when called.

Moved the filtering system from `event` to `eth/filters` package and
removed the `core.Filter` object. The `filters.Filter` object now
requires a `common.Database` rather than a `eth.Backend` and invokes the
`core.GetBlockByX` directly rather than thru a "manager".

I forgot to update one instance of "go-ethereum" in commit 3f047be5.

All code outside of cmd/ is licensed as LGPL. The headers
now reflect this by calling the whole work "the go-ethereum library".

Transaction pool will attempt to ignore invalid transactions it had
previously encountered.

* proper 0xhex
* filters fixed
* start of filter manager
* accounts for ws. Closes #246

* Fork tests (equal and larger chains)
* `chain.link` fields are now exported
* moved debug function from state to dump.go