Make MutableCompressedVectors methods return bytes allocated for simpler byte accounting logic #614

The methods for MutableCompressedVectors#encodeAndSet and MutableCompressedVectors#setZero are void. It would be helpful if they instead returned the number of bytes allocated as a result of the method invocation. The alternative solution, which I have implemented in datastax/cassandra#2144 is to call ramBytesUsed() before and after those methods, which works, but seems potentially more expensive than necessary.