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.
Change "configured" to "enabled" for clarity.
Merge branch 'develop' into prefix-compression-min
Merge pull request #624 from wiredtiger/prefix-compression-min

Add a minimum saving before prefix compression is applied to keys.