Fix some static and const correctness

Signed-off-by: Dima Kuznetsov <dkuznets@redhat.com>
Changed thread creation to thread pool.

Request handler now delegates the processing to a thread pool of
workers. If no param is specified, 0 is assumend and the thread pool in
not limited, however it may reuse threads. Otherwise, if specified,
threads are NOT created immediately, however retained and reused.

Signed-off-by: Dima Kuznetsov <dkuznets@redhat.com>
Add support for older versions of GLib2

Signed-off-by: Dima Kuznetsov <dkuznets@redhat.com>
Made max_threads private in IOProcess wrapper

Signed-off-by: Dima Kuznetsov <dkuznets@redhat.com>
Changed g_thread_pool_push error handling

Using the more portable GError param that is supported prior to 2.32
versions.

Signed-off-by: Dima Kuznetsov <dkuznets@redhat.com>
Merge pull request #1 from dimakuz/master

Added thread pool support