Add a dummy 'game_waitforblock' method that only sleeps for now, and
encapsulate the RPC server's client streams into a new class that can be
used in the future to persist the streams in new threads for
asynchronous RPC calls.

Conflicts:
	src/bitcoinrpc.h
	src/namecoin.cpp

Implement async RPC method calls that spawn a new thread waiting to
finish them.

Conflicts:
	src/namecoin.cpp

Move the check for RPC server shutdown above accept call, so that it
does execute when stop() was called.

Add a new "waitforblock" async RPC call, that currently just sleeps for
some time instead of doing actual things.

Implement the body of the waitforblock RPC call, but the condition
variable introduced is not yet notified when new blocks actually arrive.

When a new block has finished processing, notify all threads waiting on
the condition variable about it so that waitforblock now fully works.