added new op GetLastRecords
Quickfix for changing input sizes in RadixJoin

If operators of the radix join are executed interleaved with inserts on
the source tables, the input size changes. This leads to buffer
overflows if table sizes grow beyond the capacity of intermediary
results.
Revert "Quickfix for changing input sizes in RadixJoin"

This reverts commit 2ee6ada48821dfb2959e717957cc0de714da19b2.
This does not work for more than one partition.
Merge remote-tracking branch 'jwust/feature/getLastRecords' into feature/dynamic
Manually revert 4b7fa37

4b7fa37 was a bad idea. Instead we should make sure that passing mts==0
to determine dynamic count always returns 1. Only this way we can make
sure virtual operators like the radix join will be properly
instantiated.
Locking fix in DynamicTaskQueue

Tasks were not properly locked for notifications whilst scheduling was
in progress. Deadlocks ensued for dynamic scheduling scenarios.

This is a cherry-pick of the proper fix that was applied to the master.