SERVER-13495: fix ClientCursor::_pinValue concurrency
remove redundant code
small cleanup of ThreadLocalIntents::spool

Several small changes.

1) push our intent to the spool before checking if we should unspool.  just
seems cleaner.

2) the setting of _hasWritten as written isn't thread safe for non-intel
architectures, i believe, as it doesn't mem fence.  moved to _unspool() which is
inside the mutex.  in addition this is more efficient as _unspool is called 10x
less than the previous location so should be faster.

3) the old constructor was kind of weird, using vector<>::reserve() makes more
sense.
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