Gardening: Polish enum lookup in MemcacheProtocol

Motivation
----------
Simplify enum definitions and lookups.

Modifications
-------------
Use IntMap for looking up enum values by their code.

Change the enum constructors to take `int` so we can
cast (to byte or short) in just one place, instead of
having to repeat the cast in every enum value definition.

Performance Note
----------------
These enum parsing methods are currently used only by
diagnostic code, so performance isn't absolutely critical.
However, after a bit of JIT action, IntMap is measurably
faster than the switches it replaces here.

Change-Id: I83ad68b42435aa5a3ae24765b2745ae6c9a2c1bf
Reviewed-on: https://review.couchbase.org/c/couchbase-jvm-clients/+/206121
Tested-by: Build Bot <[email protected]>
Reviewed-by: Graham Pople <[email protected]>

Author: dnault