CPython: first pass at readonly txn freelist

with 2047 of 2048 reader slots allocated:

$ python -mtimeit -s 'import lmdb; e = lmdb.open("/tmp/test")' 'e.begin().abort()'
100000 loops, best of 3: 10.1 usec per loop

$ git stash &>/dev/null ; python setup.py build &> /dev/null
$ python -mtimeit -s 'import lmdb; e = lmdb.open("/tmp/test")' 'e.begin().abort()'
1000000 loops, best of 3: 0.47 usec per loop

with 0 of 2048 reader slots allocated:

$ python -mtimeit -s 'import lmdb; e = lmdb.open("/tmp/test")' 'e.begin().abort()'
1000000 loops, best of 3: 1.91 usec per loop

$ git stash pop &>/dev/null ; python setup.py build &> /dev/null
$ python -mtimeit -s 'import lmdb; e = lmdb.open("/tmp/test")' 'e.begin().abort()'
1000000 loops, best of 3: 0.472 usec per loop
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