MB-8552: Support for datatype field

- As part of extended flexible metadata
- Requires: http://review.couchbase.org/#/c/30974/

In memory::
 __________________________________________________
| key |  MetaData  (cas,exp,flags)  |   VALUE BLOB |
|_____|_____________________________|______________|

VALUE BLOB:
 __________________________________________________
| FLEX_META_CODE (1B) | DATATYPE (1B) | VALUE BLOB |
|_____________________|_______________|____________|

FLEX_META_CODE = 0x01, indicating one field in the extended
metadata section

In couchstore::

METADATA BUFFER:
 _______________________________________________________________________
| cas (8B) | exp (4B) | flags (4B) | FLEX_META_CODE (1B) | datatype(1B) |
|__________|__________|____________|_____________________|______________|

Change-Id: Ifb3bc59892130f2fddf02f2188773ba3a67f2ce1
Reviewed-on: http://review.couchbase.org/31046
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>