CollectionSyncTest: add benchmarks

This adds two banchmarks for CollectionSync: one that does an initial sync
of 60 000 Collections, and one that does incremental sync against 60 000
Collections.

On my computer, the results are around 160 seconds for benchmarkInitialSync()
and around 8 seconds for benchmarkIncrementalSync().

The benchmarks are disabled by default, because the cleanup takes insane
amount of time (I blame poor implementation of DELETE command on the server).