CCBC-528: Fix potential buffer corruption if observe body > 512b

If a single observe command exceeds 512 bytes, we might end up in a
situation where the values are silently discarded away.

This patch fixes this by replacing the ringbuffer system with the newer
and more straight-forward lcb_string (we don't need an erasable buffer).

Change-Id: I6d4ccd39659713af8c13e75abaa74038e75ae98a
Reviewed-on: http://review.couchbase.org/42287
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>