regtest : specify timestamp of created blocks

Add a third parameter to setgenerate in -regtest mode that sets the timestamp for created blocks.

Also returns the hashes of blocks created in -regtest mode instead of null (only possible in regtest mode, because the RPC calls waits until the blocks are generated before returning).

I decided to submit this as a separate pull request because I'm in the middle of writing a couple of regression tests that depend on it.