After the main test, run queries from `website.xml` in parallel using
`clickhouse-benchmark`. This can be useful to test the effects of
concurrency on performance. Comparison test can miss some effects
because it always runs queries sequentially, and many of them are even
single-threaded.

This PR adds a server level config for overriding the default max number
of threads in global thread pool that is currently allowed (10,000).

This might be useful in scenarios where there are a large number of
distributed queries that are executing concurrently and where the
default number of max threads might not be necessarily be sufficient.

This adds the `max_thread_pool_size` config to
tests/server-test.xml file.

This adds the docs for the new server level setting
`max_thread_pool_size`.

…retend to work)

They keep removing the old image every couple of weeks, and the entire
docker update fails.

…-headers

Keep aliases for substitutions in query.

In case of OSX jemalloc register itself as a default zone allocator.

But when you link statically then zone_register() will not be called,
and even will be optimized out:

  $ nm clickhouse.patched  | grep -c zone_register
  0

Fix this, by manually calling it.

v2: extern C

This fixes #9848
Also fixes #11926

remove unused headers

Fix jemalloc under OSX (by registering it as default zone explicitly)

Add concurrent benchmark to performance test

Put clickhouse-local data to /tmp by default

…t-database

Fix using current database while checking access rights.

…/20.5/11920

…f7a4489166d9ef4db68ac7d528287

Cherry pick #11920 to 20.5: Fix using current database while checking access rights.