partial eviction

thread-local buffer when possible.

frwlock

refs Tokutek/mongo#886, change get_last_key to use a callback instead of copying data, so that we don't have
mongo code freeing data that fractal tree allocated.

Add db->get_last_key() and test.
Add test

partition

eviction callback

serialization. Assume the fifo is about 1kb larger than the read buffer
(in practice, the fifo is 5 bytes smaller than the read buffer).

create_no_array() so that deserialization does not malloc/free needlessly.

logger

clock behavior more consistent with the rest of the code.

fixes #122

TXNID_NONE is not the same as (TOKUTXN) 0

omt_compress_kvspace.

This allows callers to prevent a cursor that churns through many many committed
deletes to run amok.

fixes #112