HPCC-9027 Roxie file handle errors

Roxie cache of ILazyFileIo objects had a race condition (was not linking
before checking isAlive()).

Also, the counter of how many files were active was not being properly
maintained, resulting in too many calls to the function to release inactive
files.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #4159 from richardkchapman/roxie-file-race

HPCC-9027 Roxie file handle errors

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>