memtx: allow to delay deletion of temporary tuples

Irrespective of whether there's an open read view or not, we always free
memtx tuples that come from temporary spaces immediately (see #3432).
This is acceptable if read views are only used for snapshotting or
replication, but to reuse the read view infrastructure for user read
views, we need to delay deletion of temporary tuples until all read
views that may access them have been closed.

The idea is to maintain independent lists of tuple garbage collection
arrays for temporary and normal tuples. If a read view doesn't need to
access temporary tuples, we create one garbage collection array for it,
otherwise we create two garbage collection arrays. When we free a tuple,
we choose a garbage collection array for it looking at its type.

Closes #7412

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