Limit the number of inactive handles and uvreqs kept for reuse

Instead of growing and never shrinking the list of the inactive handles (to be reused mostly on the UDP connections), limit the number of maximum number of inactive handles kept to 64. Instead of caching the inactive handles for all listening sockets, enable the caching on on UDP listening sockets. For TCP, the handles were cached for each accepted socket thus reusing the handles only for long-standing TCP connections, but not reusing the handles across different TCP streams.

Set the number of maximum free items for the uvreq memory pool to 64.

Closes #4265 (closed)