D-15000: race condition issue causing missed events

The changes in this PR are:
* get rid of the Java code that are getting timestamps during INSERTs. Instead, rely on database generated  timestamps
* during a feed head fetch, instead of fetching entries from now() or older, fetch entries that are now() - n seconds or older, where n is configurable via the feedHeadDelayInSeconds parameter in the application-context.xml.