gitaly: Add a hook before storing the log index
requested to merge 4818-add-hook-to-stop-the-transactionmanager-before-calling-storeappliedlogindex into master
In TransactionManager tests, we don't have any way to test situations
wherein a log entry has been applied, but before storing the log index,
the TransactionManager fails.
To test such scenarios, lets add a new hook beforeStoreApplied, which
allows the user to interject the TransactionManager right before
storing the log index.
Add some tests around this behavior.
Closes #4818 (closed)
Edited by Karthik Nayak