DEVEL15-windows-btree-flush-race-20090522

DEVEL15-windows-btree-flush-race-20090522

LICENSE MIT
FIXES 124787

A flush operation on a directory will result in the btree being destroyed.
This can race with on-going operations.  Make sure that the dirlock is held
if the btree is destroyed.  Otherwise, just invalidate the btree version
number.

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This delta was composed from multiple commits as part of the CVS->Git migration.
The checkin message with each commit was inconsistent.
The following are the additional commit messages.
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LICENSE MIT
FIXES 124787

correct sandbox leakage

(cherry picked from commit a31fd7f70f0049a180e5b02202941d3b1266c1f8)