Improve greylist handler

Summary:

• introduce DB transaction
• limit number of selects from greylist_connect table
• limit number of whitelist record updates

Test Plan: ./phpunit

Reviewers: #hkccp_developers, mollekopf

Reviewed By: #hkccp_developers, mollekopf

Subscribers: #hkccp_developers

Differential Revision: https://git.kolab.org/D4692