Hacked in lazily deserialized datum_t
Bug fix to datum serialization from previous commit
Another bug fix to lazy datum deserialization
Minor deserialize_lazy cleanup
Make lazy datum serialization smarter: Only do it sometimes.
Also made it backwards compatible.
Cleaned up lazy datum deserialization logic. Completely hid LAZY_SERIALIZED type from public datum_t interface.
Style fix
Merge branch 'next' into daniel_lazy_datum_deserialize

Conflicts:
	src/rdb_protocol/datum.cc
Removed unnecessary line
Use varint for serializing lazy datum size
Force full deserialization before writing to disk.
Merge branch 'next' into daniel_lazy_datum_deserialize
Minor refactoring of lazy datum serialization.
Adds unit test for lazy datum (de-)serialization
Merge branch 'next' into daniel_lazy_datum_deserialize

Conflicts:
	src/rdb_protocol/datum.cc
	src/rdb_protocol/datum.hpp
	src/unittest/datum_test.cc