Add short delay before keepalives + 'RECONNECTING'
Changed my mind - it's a good idea to wait a short time before
sending a keepalive request: this will make sure we never
tightloop.
This also adds a 'RECONNECTING' state for when a sync request has
failed but there is no reason to suspect there is actually a
connectivity problem. This is necessary for the tests to be able
to advance the clock at the appropriate time, but could be nice
for clients. Add a breaking change changelog entry since
technically this is an API change that might break clients if they
were relying on 'SYNCING' always coming before 'ERROR' for some
reason.
