Update MaxAttachmentSize option to document its other use

This dual-meaning of MaxAttachmentSize was introduced in eeb5a70, in
3.0.10.
Increase maximum session size to match maximum attachment size

Apache::Session::Oracle defaults the maximum session size to 8k, using
LongReadLen; this is insufficient for many RT sessions.  Increase the
size of Apache::Session::Oracle's LongReadLen to match LongReadLen as it
is defined in core RT -- as $MaxAttachmentSize.
Since ede2c65, Oracle has a Users1 index, not Users2
Ensure that reports' entries are sorted as expected
Provide a ticket subject to squash '' -> NULL -> undef warnings on Oracle
Switch from "use base" to a method which can run more than once

RT::Handle->FinalizeDatabaseType is called when the package is
require'd, thanks to the final lines of RT::Handle.  This means that it
is always called in RT::Test->bookstrap_db, which is always called on
'use RT::Test'.  Furthermore, "use base ..." will only have an effect
_once_.  As such, in t/web/install.t, the "SQLite" handle is actually
still an DBIx::SearchBuilder::Handle::Oracle despite DatabaseType being
SQLite.  This leads to the SystemDSN being
dbi:SQLite:sid=/path/to/rt4test_install_xxx;host=hostname;port=1521
which fares about as well as you might expect.

Switch to setting the parentage via an explicit require and @ISA, which
can be called more than once.
Merge branch '4.2/oracle-fixes' into 4.2-trunk