ReplicationHandler should throttle across all requests instead of for each client

SOLR-6485 added the ability to throttle the speed of replication but the implementation rate limits each request. So e.g. the maxWriteMBPerSec is 1 and 5 slaves request full replication then the effective transfer rate from the master is 5 MB/second which is not what is often desired.

I propose to make the rate limit global (across all replication requests) instead.