Don't start event source until Rails is ready.

[TreyE]

During some workloads, DevOps discovered that Event Source would sometimes undertake work before the Rails application had fully booted and initialized all of its items (such as database connections). This was because Event Source was creating connections and workers when the Event Source initializer file was being loaded, not after Rails had signalled it had completed the boot process.

Event source has been updated in the following way:

1. Configuration is still loaded in the initializer file, so the changes are backward compatible.
2. Connections and work are not undertaken until after the Rails framework triggers the finalizer hook indicating all frameworks and plugins have been initialized and started.

Ticket: https://www.pivotaltracker.com/story/show/188045317

[saikumar9]

lgtm! Regression testing is needed.

[raghuramg]

looks good!