(PDB-4938) Deliver promise after initial gc
(PDB-4938) Deliver promise after initial gc

Add a promise to shared-globals that is delivered after the initial
garbage collection finishes. On the PE side, we will wait on that
promise to start the initial sync.

puppetlabs/puppetdb,puppetlabs/puppetdb,puppetlabs/puppetdb,puppetlabs/puppetdb,puppetlabs/puppetdb