an atomic tag-update function

[Krastanov]

Frequently we do

slot, tag = query(...)
pop!(slot, tag)
tag!(slot, tag)

This is fine as long as the pop! and tag! are next to each other, but it is prone to race conditions if there is a @yield between them. An atomic version of this operation would be very valuable.

[Krastanov]

we have querydelete! in #81 , but there is no an atomic queryupdate! function yet