Bug 4156: comm.cc "!commHasHalfClosedMonitor(fd)" assertion (#1443)

Bug 4156: comm.cc "!commHasHalfClosedMonitor(fd)" assertion (#1443)

This bug is specific to "half_closed_clients on" configurations.

    assertion failed: ... "isOpen(fd) && !commHasHalfClosedMonitor(fd)"
    location: comm.cc:1583 in commStartHalfClosedMonitor()

Squid asserts because Server schedules comm_read() after receiving EOF:
That extra read results in another EOF notification, and an attempt to
start monitoring an already monitored half-closed connection.

Upon detecting a potentially half-closed connection,
Server::doClientRead() should clear flags.readMore to prevent Server
from scheduling another comm_read(), but it does not and cannot do that
(without significant refactoring) because

* Server does not have access to flags.readMore
* flags.readMore hack is used for more than just "read more"

We worked around the above limitation by re-detecting half-closed
conditions and clearing flags.readMore after clientParseRequests(). That
fixed the bug but further increased poor code duplication across
ConnStateData::afterClientRead() and ConnStateData::kick() methods. We
then refactored by merging and moving that duplicated code into
clientParseRequests() and renamed that method to make backports safer.