Remove global mutable serializer registry

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.
Just thinking, this is the only use of MutableDynamicSerialization and it doesn't make much sense (because there's no guarantee we register serializations before we get the corresponding event) so in a future PR I think we might want to change this SbtClient API and then consider combining all of DynamicSerialization, ImmutableDynamicSerialization, and MutableDynamicSerialization into a single class. I think this PR will already conflict with the SbtChannel one in a mild way though so don't want to go to town yet.