Within ContainerBroker, at various places, for example get_db_state()
and sharding_initiated(), they query the number of shard ranges of
this container by pulling out all shard ranges from shard range table,
instantiating ShardRange objects and then counting how many they are.
Those operations are very expensive, and causing HEAD/GET into large
containers to be very slow.
Instead, this patch only checks if there is any qualified shard range
exists in the shard table with optimized SQL query, which can be very
fast. On a container server setup which serves a container with ~12000
shard ranges, this patch alone improves HTTP HEAD request rate by ~10X,
and improves HTTP GET request rate by ~2X; and together with other
optimizations (patch 888310 & 888575) targeting to fix similar problems,
strong synergistic effects are seen to bring total ~22X improvement to
HTTP HEAD and ~27X to HTTP GET request rates.
Change-Id: I01fd4f3e395c8846280f44e17a56935fc6210444
bfbe8f909f