This commit includes concurrency control for datapages, the new backpressure for the c0-c1 merger, and (untested) support for defining NO_SNOWSHOVEL, and using the old approach.
There are a number of important limitations:
- statistics gathering is somewhat messed up
- the c0-c1 merge thread polls for new data
- on a sequential write, if the writers stall, the merge thread could empty memory and initiate an unnecessary merge. Instead, we should never finish a merge if c0 is less than N% full
- this commit introduces yet another batch of nasty mutex contention issues
git-svn-id: svn+ssh://svn.corp.yahoo.com/yahoo/yrl/labs/pnuts/code/logstore@955 8dad8b1f-cf64-0410-95b6-bcf113ffbcfe
