Implement pause/unpause functionality for plasma storage backend

Currently, indexing supports pause/unpause only for MOI storage backend.

With heavy DGM scenarios (like less than 5% index resident ratio), indexer with plasma backend can get really slow and will start manifesting functional issues. The situation will be worsened if more mutations keep coming in. Typically very less resident ratio situations are known to be observed due to combination of (1) Wrong sizing, (2) Non-optimal index creation and (3) Very high data ingestion rate.

Indexing infrastructure supports DDLs in paused state so that users can come out of the situation by correcting the errors.

Plasma storage backend, unlike MOI, performs various background activities like page evictions, compaction etc, which require their own memory. So, pause/unpause needs to be supported by plasma storage backend as well as indexing so that SLAs similar to MOI storage backend will be guaranteed.