Add option to control constraint check time.

By default, this is DEFERRED_WITH_RANGE_CACHE, because that's almost always the right choice for bulk loads (in 2.0).

With this (and the associated FDB branch) and fdbserver and client from default on perf02, loading a 1M file:
* immediate 150.772
* deferred 60.118
* deferred_with_range_cache 39.782