[DocDB] Prevent user writes if we are close to disk full by rejecting TabletServiceImpl::PerformWrite

[yugabyte-ci]

Jira Link: DB-11337

Customers can load data into the system until they run out of disk space. Once we run out of disk the tservers will FATAL until extra disk is added.
A simple solution that will cover the majority of cases (not all) will be to check the available disk at TabletServiceImpl::PerformWrite and reject incoming writes if we do not have sufficient space (FLAGS_reject_writes_min_disk_space = 300MB).

This will cover the majority of cases since all nodes more of less have the same size and data distribution.