Reduce lock contention in kernel port subsystem.

* Replace ports list mutex with R/W-lock.

* Move team port list protection to separate array of mutexes.
  Relieve contention on sPortsLock by removing Team::port_list from its
  protected items. With this, set_port_owner() only needs to acquire the
  sPortsLock for reading.

* Add another hash table holding the ports by name. Used by find_port()
  so it doesn't have to iterate over the list anymore.

* Use slab-based memory allocator for port messages. sPortQuotaLock was
  acquired on every message send or receive and was thus another point
  of contention. The lock is not necessary anymore.

* Lock for port hashes and Port::lock are no longer locked in a nested
  fashion to reduce chances of blocking other threads.

* Make operations concurrency-safe by adding an atomically accessed
  Port::state which provides linearization points to port creation and
  deletion. Both operations are now divided into logical and physical
  parts, the logical part just updating the state and the physical part
  adding/remove it to/from the port hash and team port list.

* set_port_owner() is the only remaining function which still locks
  Port::lock and one or two of sTeamListLock[] in a nested fashion.
  Since it needs to move the port from one team list to another and
  change Port::owner, there's no way around.

* Ports are now reference counted to make accesses to already-deleted
  ports safe.

* Should fix #8007.
Move B_MOUSE_IDLE generation to app_server.

* BWindow used to generate the B_MOUSE_IDLE events by sending a
  delayed message with a one-shot BMessageRunner to itself.
  Every creation and deletion of BMessageRunners causes synchronous
  messaging between the application under the mouse cursor and the
  registrar. This creates large amounts of calls to set_port_owner()
  in the kernel whenever moving the mouse.

* Now, B_MOUSE_IDLE is sent by the cursor loop inside the app_server
  instead. When the mouse wasn't moved for the tooltip delay time,
  it inserts a B_MOUSE_IDLE message into the event stream.

* The tooltip delay thus becomes a system-wide constant and is not
  configurable per-application anymore (no code currently in the
  Haiku repo makes use of that anyhow).