Reduce threading in market.py

### Motivation
The [sqlite3](https://docs.python.org/2.7/library/sqlite3.html?highlight=sql#multithreading) module doesn't support multithreaded access to the DB, but we 've been doing just that since #608. This may lead to crashes, DB corruption and lots of other possible issues. There might be a way to reconcile concurrency and `sqlite3` (via locks, etc) but it isn't obvious to me. So we 're back to serialized access.

### Changelog.
* Serialize `node/market.py::Market.save_contract` for the time being.