[FAB-7997] Fix statecouchdb: rm extra loop & comt.

Addressed review comments given in
https://gerrit.hyperledger.org/r/#/c/16695/

(i) remove an extra loop in LoadCommittedVersions() which is
unnecessary.
(ii) add additional comments for LoadCommittedVersions()
(iii) some code refactoring as well

Change-Id: I39b18cdcbad3f3e8e6b02b22d79ca66031a86958
Signed-off-by: senthil <cendhu@gmail.com>

Author: cendhu