Ftruncate is incompatible with file mapping on Windows, return EBUSY from
the OS layer if a file handle has a mapping and ftruncate is called.

Change exclusive operations to close all open connection handles for a file
object before performing the operation.

Reference #1254.
Add a couple of comments about truncate now failing, restructure the extent
list truncation code: we don't much care if the truncate fails, we want to
reset the logical end of the file anyway.
Rename the WT_FH.refcnt field to be WT_FH.ref to match the new
reference-count mapped field name.
Merge pull request #1262 from wiredtiger/truncate-with-mapping

Truncate with mapping