feat: unprepare statement references after sql execution

Picodata SQL now manages the Tarantool statement cache using a
dedicated SQL fiber that handles preparation and unpreparation of
statements based on an LRU eviction policy. Prepared statements
can be executed across different sessions by SQL clients.

Previously, when a client executed a prepared statement, it
increased the reference count in the statement cache and linked
the statement to the client's session. While this approach seemed
fine, it caused issues during eviction, as references to these
statements remained in client sessions, preventing proper eviction.

This commit addresses the issue by ensuring that if a statement is
added to the current session during execution, it is removed and
the session state is restored once execution is complete.

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NO_CHANGELOG=internal