Optimize -regtest setgenerate block generation

Speed up generating blocks in regression test mode, by moving
block-creating and nonce-finding directly into the setgenerate
RPC call (instead of starting up a mining thread and waiting for
it to find a block).

This makes the forknotify RPC test three times quicker, for
example (10 seconds runtime instead of 30 seconds, assuming
the initial blockchain cache is already built).
regression test only setmocktime RPC call
RPC tests: create initial chain with specific timestamps

Use setmocktime to create the initial block chain with
10-minute-apart-blocks starting 1 Jan 2014.
python-bitcoinrpc: Better batch support