Limit Tserver Write Threads

Tservers cannot be configured to limit how many write threads they will create. It would be very helpful if an upper limit could be specified given that there are limits in how quickly the Tserver can make WAL updates in HDFS, for example. Rather than collecting threads and subsequently blocking them, it would be helpful if clients received a failure because a Tserver has already reached its maximum.

No files or tests are named. Start by locating the Tserver write-thread creation and the WAL update path involving HDFS, then determine where a configurable upper limit belongs. Done means clients receive a failure when the configured limit is reached rather than waiting for another write thread.

Written by the indexing model from the issue text.