memtx: refactor tuple read view API

This commit replaces enter/leave_delayed_free_mode() MemtxAllocator
methods with open/close_read_view(). The open_read_view() method
returns a pointer to the new read view, which is supposed to be pssed
to close_read_view() when the read view is no longer needed. The new
API will allow us to optimize garbage collection, see #7185.

Currently, open_read_view() always returns nullptr and both ReadView
and memtx_read_view_opts are empty structs. We will extend them in
the future.

Closes #7364

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring