Make waiting change listener trigger only when relevant channels change
Added ability to import pre-existing docs in bucket

Adding the db config property "importDocs":true will scan the database
at startup for documents that aren't known to the gateway (i.e. docs
whose keys don't start with "_sync:" and don't have a "_sync" property)
and add them as though they were new documents. This will add a "_sync"
property, run the doc through the sync function, and assign channels
and user/role access as usual.
This should only be used on a single gateway node, otherwise all nodes
will simultaneously try to import documents and step on each other.