Prefix compression min

Add the WT_SESSION.create prefix_compression_min string configuring the number of bytes that must be gained before prefix compression activates, with a default of 4.

Quit documenting the WT_SESSION.key_gap configuration string: it's only used when instantiating keys as the result of a reverse cursor scan, I don't think it's interesting enough to surface to applications, at this point.

Ref: #622.