* internal/databroker: make Sync send data in smaller batches
GRPC streaming is better at sending multiple smaller message instead of
a big one.
Benchmark result for sending 10k messages at once vs multiple batches,
each with 100 messages:
name old time/op new time/op delta
Sync-12 14.5ms ± 3% 12.4ms ± 2% -14.40% (p=0.000 n=10+9)
* cache: add test for databroker sync
