Tag context should not be persistent accross commands

Unlike with SELECT for collections, we have no way to change tag context
or unset it, so if one command sets the context, all subsequent commands
would have to override it.

Instead we make tag context non-persistent (it's reset before every new
handler is started).

Once (if) we kill SELECT in KF5 and specify the context in each command
instead for collections too, we can make the entire context to be per-
command instead of per-connection.