[BradAF]

When a client disconnects, its socket can be disposed while output is
still queued for the session. Send() caught the ObjectDisposedException,
logged a warning and returned, so the session never reached LoggedOff
and ProcessDisconnects() never freed its channel. Every later queued
write logged the same warning again, and the user saw "already logged
in" when reconnecting.

Call CloseSocket() instead, which moves the session to LoggedOff so the
channel is reclaimed on the next ProcessDisconnects() pass. It is
guarded by _socketClosed, so repeated calls are harmless.

Channels take the lowest free number and are freed in any order, so the
channel numbers in use can have gaps (for example 0 and 2, with nothing
on 1). RemoveSessions() looped i from 0 to Count - 1 and treated i as a
channel number, so a channel numbered Count or higher was never checked.
A logged-off session on channel 2 with only two sessions open was
skipped on every pass, leaving the channel occupied and causing "already
logged in" on reconnect.

Loop over the channel numbers actually in use instead, still removing
the highest first.