Stability bugfixes for online delete SHAMapStore
The correct ledger age is necessary for checking health
status, and the previous behavior caused the online deletion process to
abort if the process took too long.

The tuning parameter added and the parameter whose default was modified both
minimize impact of SQL DELETE operations by decreasing the default batch size
for deletes and for increasing the backoff period between deletion batches.
These parameters decrease contention for the SQLite and I/O with the trade-off
of longer processing time for online delete. Online-delete is not a
time-critical function, so a little slowness in wall-clock time is not harmful.