Send the proper "hello" message to HBase 0.95 and up.

Upon connecting to HBase we need to send a "hello" message, which
is now partly a protocol buffer.
Remove some unused helper code to create buffers.

We never need to use dynamic buffers as we can always produce a
good conservative estimate of how many bytes we need to serialize
an RPC.
Update the per-RPC header for HBase 0.95+.
Refactor how RPC objects know what RPC method they're for.

Introduce a new internal method to HBaseRpc, method(version), which
must return the RPC method name to invoke, given the HBase protocol
version of the server we're connected to.

Previously, all HBaseRpc instances had a `method' byte array in the
object that carried the name of the method to invoke (e.g. "get" or
"put").  Although the names were only created once (statics) this
still added one reference to each and every RPC object.

With HBase 0.95+ compatibility, many RPCs have changed their names
and thus it makes sense to standardize on an internal API that RPCs
must have to declare the name of the RPC method they should invoke
as a function of the HBase protocol version.
Compile Java code generated from protobuf files separately.

There is so much code in there that's a bit slow to compile,
even though javac is far from being a real compiler.  Since
the generated protobuf can be compiled on its own and is less
likely to change (as it would only change as a result of an
update made to one of the .proto files), building it on its
own allows subsequent incremental builds to stay just as fast
as they used to be.
WIP: Convert META lookups to protobuf.