have an auto-commit parameter for dumb clients

[andreasbaumann]

We have explicit transactions.
If a client tries to feed us too much data, we should have a protection mechanism, sort
of a forced auto-commit after N documents. Also the default commit-size 1 for auto-commit
should be made configurable.

[andreasbaumann]

A configuration option like in SolR: auto-commit after N documents or better after using N bytes
of memory for a transaction. But for the latter we need some support from the strus library telling
us about its memory usage.

[andreasbaumann]

On the other hand: a dumb client will continue to feed us documents for a now forcefully
closed transaction. But at least the server is protected.