Allow files.rm_recursive to yield whenever possible.
Allow files.rm_recursive to yield whenever possible.

A while back, for performance reasons, we disabled yielding for all
files.* operations unless METEOR_DISABLE_FS_FIBERS was set to false.

This was safe for almost all files.* operations, because most of them have
a synchronous fs.*Sync version available.

For a more complicated operation like files.rm_recursive, however, there
is no synchronous or asynchronous counterpart in the fs.* namespace, so
the safety of disabling fibers is not guaranteed.

Lately, files.rm_recursive has become a major source of uncaught ENOTEMPTY
errors on Windows, because rimraf.sync fails with that error, and we don't
give files.rm_recursive_async a chance to delete the directory in a more
persistent, forgiving manner.

The only reason we haven't been falling back to files.rm_recursive_async
is that YIELD_ALLOWED is false by default, so canYield() returns false.

This commit distinguishes between canYield() and mayYield(), and uses
canYield() in files.rm_recursive to determine whether it is technically
safe to yield, regardless of YIELD_ALLOWED.

Anyone who ever asked "Can I go to the bathroom?" in elementary school,
only to be mercilessly rebuked with "I don't know, CAN YOU?" should
understand the difference between these two functions.

Hansoft/meteor,Hansoft/meteor,Hansoft/meteor,Hansoft/meteor,Hansoft/meteor,Hansoft/meteor,Hansoft/meteor