Improve ability of checkpoint memory recovery

There are a number of small changes that we need to put in for making memory recovery from checkpoints more effective. I use this MB for tracking them all.
Current list:

• MB-47386 now allows to increase the max num of checkpoints from 2 to a much higher number
• Triggers for when creating a new checkpoint needs to be adjusted - for example, currently we trigger checkpoint creation based on max num of items for a checkpoint (defaulted to 10k), while a mem-usage related metric would certainly be more appropriate
• Consider removing call to VBucketMap::getVBucketsSortedByChkMgrMem from CheckpointRemover - call is O(N) in the size of the VBMap
• The order on how we attempt mem-recovery is currently expel->cursor_drop->ckpt_rem, while we should first try cktp_rem as some checkpoints may be eligible for removal even without dropping any cursor