Function steal-cache-table was made thread-safe.

[svetlyak40wt]

Previously it might modify hash table when other thread might do cleanup-cache-pool.

[svetlyak40wt]

@fukamachi please, review this small change. There is still the problem when two new threads are starting to access database simultaneously, because the might try to insert entries into unprotected hash-map (line 21) in parallel.

[fukamachi]

Thanks!

[svetlyak40wt]

Horay!