cloud_storage: noisy 'hydrating segment' log messages even when already in cache

[jcsp]

I get messages like this on read, even if the segment is already in cache. This should probably be at debug level.

INFO  2021-12-17 13:35:47,838 [shard 1] cloud_storage - [fiber4~204 kafka/verifier/7] - remote_segment.cc:166 - Start hydrating segment {"2aba51fd/kafka/verifier/7_185/401743-1-v1.log"}, 1 consumers are awaiting

The message when we actually hydrate a segment is similar but different. This seems like a totally valid thing to log at info level.

INFO  2021-12-17 13:35:47,838 [shard 1] cloud_storage - [fiber4~204 kafka/verifier/7] - remote_segment.cc:185 - Hydrating segment {"2aba51fd/kafka/verifier/7_185/401743-1-v1.log"}

Also messages like this, which seems like it should probably be at debug level.

INFO  2021-12-17 13:35:48,127 [shard 1] cloud_storage - [fiber13~120 kafka/verifier/8] - remote_segment.cc:222 - Hydraton loop is stopped

[jcsp]

The "Start hydrating" message is addressed in #3280