Introducing app joint to accurate app release.

For accurate app descriptor release, it is required to count the number of
use counts.  Use count increased when:
- app linked to configuration app queue;
- socket conf stores pointer to app;
- request for start app process posted to router service thread;

Application port has pointer to app, but it does not increase use count
to avoid use count loop.

Timer needs a pointer to nxt_timer_t which is stored in engine timers tree.
nxt_timer_t now resides in nxt_app_joint_t and does not lock the application.

Start process port RPC handlers is also linked to nxt_app_joint_t.

App joint (nxt_app_joint_t) is a 'weak pointer':
- single threaded;
- use countable;
- store pointer to nxt_app_t (which can be NULL);

nxt_app_t has pointer to nxt_app_joint_t and update its pointer to app.

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