Fix thread counter leak

[amosbird]

This issue has appeared three times in kuaishou (every 1-2 months). TCPServerDispatcher fails to create any new threads because _currentThreads is leaked.

The issue is introduced in pocoproject#1965 which was actual to fix another _currentThreads leaking issue... However I cannot find the related commit in current poco's master or develop branch. Perhaps it's already been reverted.

[alexey-milovidov]

CC @akuzm

[amosbird]

I've successfully reproduced the issue with gdb. Here are the steps:

1. Set a breakpoint at https://github.com/ClickHouse-Extras/poco/blob/clickhouse/Net/src/TCPServerDispatcher.cpp#L105
2. Submit three queries select 1 via HTTP. Now we have three suspended threads at the above breakpoint. Let's denote them as A, B and C.
3. Step both A and B right before https://github.com/ClickHouse-Extras/poco/blob/clickhouse/Net/src/TCPServerDispatcher.cpp#L123 , that is, we successfully destruct ThreadCountWatcher but without decrementing _currentThreads because the _queue is not empty.
4. Step C to dequeue the last query. Now we have an empty queue.
5. Continue both A and B. They'll break the main loop and exit (in the sense of Poco, they'll be cached)

Now we have _currentThreads = 3 but there is only one thread left.

[alexey-milovidov]

Sorry, we almost missed this PR.