MB-7685: Refactor VBucketDeletion task

Change-Id: I021ca96c0e2e914ea498e4d2d78b9477ac3b5b79
MB-7685: Refactor the StatsSnap task

Change-Id: I22c46bed3da65b022a76bf756715ff475bc70617
MB-7685: Refactor MutationLogCompactor task

Change-Id: I21de542882121e43dc8764e139c089c7c51c7d1b
MB-7685: Remove concurrency related parameters from couchstore

Thes parameters will be decided at a higher level in the future
so it doesn't make sense to have them anymore. I am also not
aware of anyone who would want to use them or does currently
use them.

Change-Id: I117fe499dac868352084854ae1cdad992971be76
Add new iomanager

Change-Id: Iccaa2c027cdb9abdd6c580973f94ee6d254ef035