http2: interface to support pluggable schedulers

This adds an interface to support pluggable schedulers. The interface
is defined in writesched.go. The only type needed by this interface is
FrameWriteRequest, which describes a request to write a frame (this used
to be called frameWriteMsg). The scheduler can be configured with a new
field in http2.Server. Two schedulers are implemented:

1) A random scheduler that is essentially equivalent to the existing
   scheduler. This is currently the default scheduler if none is
   configured. The implementation is in writesched_random.go.

2) A scheduler that uses H2 weights and priorities. The H2 priority tree
   is maintained as a tree of priorityNodes. The next frame is chosen by
   walking this tree in topological order, where sibling nodes are ordered
   by their bandwidth usage relative to their H2 weight. Two optional
   features are added to improve performance -- these are configured with
   PriorityWriteSchedulerConfig.

Fixes golang/go#16168

Change-Id: I97ec93e5c58c2efec35455ba2f3c31e849f706af
Reviewed-on: https://go-review.googlesource.com/25366
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

Author: tombergan