Allows the user to pass null as the second or third parameter
to signrawtransaction, in case you need to (for example) fetch
private keys from the wallet but want to specify the hash type.
Change internal HTTP JSON-RPC server from single-threaded to
thread-per-connection model. The IP filter list is applied prior to starting
the thread, which then processes the RPC.
A mutex covers the entire RPC operation, because not all RPC operations are
thread-safe.
[minor modifications by jgarzik, to make change upstream-ready]