The API is simplified: do not expose the user code to several
queues.  A "connection queue" is now an internal concept.
The user processes connections using the single function
lsquic_engine_process_conns().  When this function is called,
only those connections are processed that need to be processed.
A connection needs to be processed when:

    1. New incoming packets have been fed to the connection.
    2. User wants to read from a stream that is readable.
    3. User wants to write to a stream that is writeable.
    4. There are buffered packets that can be sent out.  (This
       means that the user wrote to a stream outside of the
       lsquic library callback.)
    5. A control frame (such as BLOCKED) needs to be sent out.
    6. A stream needs to be serviced or delayed stream needs to
       be created.
    7. An alarm rings.
    8. Pacer timer expires.

To achieve this, the library places the connections into two
priority queues (min heaps):

    1. Tickable Queue; and
    2. Advisory Tick Time queue (ATTQ).

Each time lsquic_engine_process_conns() is called, the Tickable
Queue is emptied.  After the connections have been ticked, they are
queried again: if a connection is not being closed, it is placed
either in the Tickable Queue if it is ready to be ticked again or
it is placed in the Advisory Tick Time Queue.  It is assumed that
a connection always has at least one timer set (the idle alarm).

The connections in the Tickable Queue are arranged in the least
recently ticked order.  This lets connections that have been quiet
longer to get their packets scheduled first.

This change means that the library no longer needs to be ticked
periodically.  The user code can query the library when is the
next tick event and schedule it exactly.  When connections are
processed, only the tickable connections are processed, not *all*
the connections.  When there are no tick events, it means that no
timer event is necessary -- only the file descriptor READ event
is active.

The following are improvements and simplifications that have
been triggered:

    - Queue of connections with incoming packets is gone.
    - "Pending Read/Write Events" Queue is gone (along with its
      history and progress checks).  This queue has become the
      Tickable Queue.
    - The connection hash no longer needs to track the connection
      insertion order.