todo

memory reclamation
------------------
- allow threads to dynamically enter and exit rcu's token passing ring
- augment rcu with heartbeat manager to kill and recover from stalled threads
- make rcu try yielding when its buffer gets full
- use alternate memory reclamation schemes: hazard pointers and/or reference counting

quality
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- verify the memory management of keys in list, skiplist, and hashtable
- transaction tests
- validate function arguments in interface functions
- document usage
- document algorithms

optimization
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- investigate 16 byte CAS; ht can store GUIDs inline instead of pointers to actual keys 
- txn write after write can just update the old update record instead of pushing a new one
- use a shared scan for write-set validation in txn, similar to ht copy logic
- experiment with the performance impact of not passing the hash between functions in ht
- experiment with embedding the nstring keys in the list/skiplist nodes
- lower skiplist's high_water when the actual number of levels in use drops
- non-power-of 2 sized hashtables for improved memory usage
- mem2

features
--------
- allow values of 0 to be inserted into maps (change DOES_NOT_EXIST to something other than 0)
- read-committed type transactions
- recycle free regions across size-classes and between threads