lib: call rmdir(2) and unlink(2) in worker threads during purging directory

purge_directory() can cause amount of disk I/O because of rmdir(2)
and unlink(2). Because they can slow down main thread significantly,
it should be done in worker threads for avoiding long request
blocking.

Cc: Philip Crotwell <crotwell@seis.sc.edu>
Cc: Masahiro Tsuji <tuji@atworks.co.jp>
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>