Remove global mutable serializer registry

[havocp]

This is the logical extension of earlier work to always pass the Writes around (and to pass only Writes, not the whole Format).

With those earlier cleanups it isn't such a big deal to pass the serialization registry around where it's needed too.

[jsuereth]

Generally LGTM

[jsuereth]

You're right, this conflicts now and needs to be refreshed.

[havocp]

Merged, and cleaned up a bit. I sorted out the race with registration in SbtClient because I needed to shuffle that code anyhow to make the channel have the serializations. Two additional pieces of work that might be nice:

• allow SbtClient.watch to take a deserializer to use only for that watcher, which decouples deserializing a bit from SbtClient construction
• break up DynamicSerialization so we have DynamicReads, DynamicWrites, and DynamicFormat.

I think we can push both of those into the future sometime.

[havocp]

Re-merged and travis passing.