[C#] Allow log compaction to non-record-boundary addresses

[badrishc]

session.Compact can now accept addresses that are not at strict record boundaries. It will return the actual address until which compaction was completed (next nearest record boundary).

For example, you can compact 20% of the log as follows:

  long compactUntil = store.Log.BeginAddress + (store.Log.TailAddress - store.Log.BeginAddress) / 5;
  compactUntil = session.Compact(compactUntil, shiftBeginAddress: false);
  await store.TakeHybridLogCheckpointAsync(CheckpointType.FoldOver);
  store.Log.ShiftBeginAddress(compactUntil);

See documentation here.

Also, added option to ShiftBeginAddress, to snap given address to nearest earlier page start.