Use maxWriteBatchSize from ismaster for write command batch splitting.

Use a default maxWriteBatchSize of 1000 while disconnected or if maxWriteBatchSize is not reported in ismaster. Otherwise, set client.max_write_batch_size to the value in ismaster, for all client types.

When streaming operations with a new write command, finish a batch after every maxWriteBatchSize documents or when exceeding maxMessageSize, whichever comes first.

Remove the 1000-operation limit from the Bulk API implementation since the underlying write command implementation now handles it.

Bulk inserts with OP_INSERT are unaffected.