Fix the Heisenbug!

push_block calls now(), which yields on its first invocation, but
push_block is a non-premptable function. To resolve this, we add a call
to now() at the beginning of chain_database::open, to ensure that now()
has already been called by the time push_block runs.
Add block limit to blockchain_market_order_history

It is unfun to sit there while the history function hunts through 200000
blocks of chain, trying to find orders. Let's not do that unless we
really, really want to.
Merge branch 'develop' of github.com:BitShares/bitshares_toolkit into develop