Allow to disable ledgers operation throttling

In the bookie client there is a throttling mechanism that is rate-limiting the number of writes/s done on a particular ledger. The default value is 5000 entries/s written on a ledger.

Trying to write faster will block the calling thread, which may not be desirable in an asynchronous server.

We should be able to disable the throttling, if the application wants to control the rate with different mechanisms.