MB-10045: Refactor encoding/decoding of UPR commands

The UPR command parsing, encoding/decoding is now in separate
modules. This way you could use them outside of the higher-level
APIs as they are defined by couch_upr.

They have been separated for the consumer and producer side as
for communication with an UPR stream you would only need the
consumer side.

This is a pure refatoring moving functions around, there's no
functional change.

Change-Id: I4e335dc5c299be3b45de0f2ed8c5a27918fb1c6a
Reviewed-on: http://review.couchbase.org/32918
Tested-by: Volker Mische <volker.mische@gmail.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>