progress on segment merge
separate lock-like thing to prevent a segment from being part of more than one merge.  commitmerge in IWriteLock.  keep written-but-not-committed merges in pendingMerge collection.  rename prependsegments to commitsegments.  move toward less Task and more Async.
rm dead code
getting setup to write xUnit tests in f#
porting current contents of tapp to F#
fsTapp mostly okay now