[Tracker] Enable a dormant feature - optimize commits for expired keys

[srieteja]

Is your feature request related to a problem? Please describe.

Feature in #1660 has been implemented in #1713 and #1727. To preserve backwards compatibility, this feature will lie dormant for now. As a response to the changes in at_server, some changes have been made in at_client in atsign-foundation/at_client_sdk#1187.

Describe the solution you'd like

Enable the dormant feature at an appropriate time. The only change required would be to change the value of the variable in AtSecondaryConfig named '_optimizeCommitLog' and set it to true.

[srieteja]

Discovered a minor issue in the client side deleteExpiredKeys scheduled job. More in atsign-foundation/at_client_sdk#1350. Carrying it forward to PR92.