Made term_t::eval be const.
Merge branch 'sam_2156_env_3' into sam_2156

Conflicts:
	src/rdb_protocol/func.hpp
	src/rdb_protocol/op.cc
	src/rdb_protocol/op.hpp
	src/rdb_protocol/term.hpp
	src/rdb_protocol/terms/datum_terms.cc
	src/rdb_protocol/terms/db_table.cc
	src/rdb_protocol/terms/error.cc
	src/rdb_protocol/terms/http.cc
	src/rdb_protocol/terms/js.cc
	src/rdb_protocol/terms/obj_or_seq.cc
	src/rdb_protocol/terms/random.cc
	src/rdb_protocol/terms/rewrites.cc
	src/rdb_protocol/terms/time.cc
	src/rdb_protocol/terms/type_manip.cc
	src/rdb_protocol/terms/var.cc