Finish 64-bit time conversion, modify protocol

Finish 64-bit time conversion, modify protocol

Convert the remaining uses of 32-bit quint-based
"[to|from|set]Time_t()" to the modern 64-bit qint64-based
"[to|from|set]MSecsSinceEpoch()", handling the Y2038 problem while
also gaining higher precision in some places.

For places where seconds are needed, e.g. converting incoming time
from the IRC protocol, make use of "[to|set|from]SecsSinceEpoch()"
instead.  As this was only added in Qt 5.8, use the "QT_VERSION"
macro for backwards compatibility, manually dividing by 1000 for the
"MSecs" function equivalents.

Beware, all new code should use "[to|from|set]MSecsSinceEpoch()" and
NOT use "[to|from|set]Time_t()", or you'll re-introduce Y2038 bugs!

Modify protocol to rename "LongMessageTime" to "LongTime", and
send/receive all times as "qint64" instead of "quint64" or "quint".
"LongMessageTime" was never in Quassel git master, so renaming it
should have no impact.

Modify Event timing to be 64-bit compatible, too.

"IrcUser::lastAwayMessage()" will be fixed in a follow-up commit.

SQLite millisecond precision will be fixed in a follow-up commit.

See https://doc.qt.io/qt-5/qdatetime.html
And https://doc.qt.io/qt-5/datastreamformat.html
And https://doc.qt.io/qt-5/qdatetime-obsolete.html