MB-7415: fast doc lookups using doc ptr instead of id

This commit is a performance optimization to speed up fetching docs.

Today XDCR uses {key, rev} pair to fetch document from storage layer,
which may incur an index lookup followed by disk lookup. However, the
location to document body is already stored in the doc_info and there
is no need to look up the index first. This commit will allow XDCR to
fetch the document directly from the pointers stored in doc_info.

Change-Id: Ie665e8c80c88dc960e1e0e135aa933ac1ba4d668
Reviewed-on: http://review.couchbase.org/24392
Reviewed-by: Junyi Xie <junyi.couchbase@gmail.com>
Tested-by: Junyi Xie <junyi.couchbase@gmail.com>
Reviewed-by: Damien Katz <damien@couchbase.com>