Implement guaranteed writes to specified number of racks

[ivankelly]

JIRA: https://issues.apache.org/jira/browse/BOOKKEEPER-995

Reporter: Rithin Shetty

With rackaware ensemble placement we get guaranteed writes to at least 2 different racks for every write quorum. We want to extend this such that user can ask for writes to at least 3 or more racks per write quorum. This is useful to ensure higher availability when one of the racks is already down: the remaining copies are guaranteed to be on different racks and hence can with stand another rack failure assuming the write quorum is 3.

[sijie]

This has been fixed by #2095