Reverts part of commit c40066173f4f536b6fa06127d4d38c5c1b33fe63 -- the part involving changing datum_t::cmp.

Conflicts:

	src/rdb_protocol/datum.cc
Used std::string::compare, wrapped in signum functions.
Removed signum wrapping, made users of cmp functions handle the return value correctly.
Undid removal of "!= 0" that happened in the recent reversion of datum_t::cmp changes.
Merge branch 'sam_1441' into next