[BOOKKEEPER-TOOL] Fix update rate-limiting for update-ledger command

[BOOKKEEPER-TOOL] Fix update rate-limiting for update-ledger command

### Motivation
Right now, `UpdateLedgerCmd` provides option `updatespersec` to throttle number of writes on zk. However, it throttles number of reads instead writes. Because of that it takes long time to complete this command as it slows down the read instead applying throttling while writing to zk.

### Modification
- Apply throttling while updating zk.
- If writes are being throttled then we also want to avoid accumulating reads so, added `maxOutstandingReads` option to manage max concurrent reads.

**Note:** We also need this change for #2321 .. so, I will rebase #2321 once this PR is merged.

Reviewers: Enrico Olivelli <eolivelli@gmail.com>

This closes #2336 from rdhabalia/update_rate