This should greatly reduce the number of unecessary sync calls by
storing the last time a gertty queried for changes to a project.
Previously, we used the updated time of the latest change in a
project, however, subsequent queries with that value would typically
return the same change even though it needn't be synced. Adjusting
that value by a small amount is unlikely to work reliably because
the query is for a relative time and it takes some time to process.
Adjusting for a larger amount (eg, a few seconds) might miss data.
Clock skew is also a concern in this system because we are using
subtracting the server time from the client's time.
By storing the last sync time locally, we can continue to update
it past the highest value that gerrit has, so that we eventually
get queries which return no results. Clock skew is not an issue
because the delta arithmetic only involves client generated times.
We can also increase the window slightly to account for query
processing time without continuously sync already-synced changes.
Change-Id: I8cd0af9bd4d3669f436f169059e4b602d4d3036c
00d3f6dbfd