Improve caching object store performance

[gruuya]

List of improvements:

• move out the writing of data to disk to a separate task
• and remove the lock on the file manager
• but let all the incoming calls for the requested range use the in-memory bytes until the write completes
• finally, change moka eviction policy to LRU

Tested on local minio using ClickBench q19, seems to improve against both old-cache and no-cache setups.