ipsec: T8975: lock vti-up-down state DB to prevent lost updates under concurrent rekey

The vti-up-down hook and vpn_ipsec.py modify a flat-file DB
(/tmp/ipsec_vti_interfaces) through three context managers that do an
unlocked read-modify-write. During a coordinated rekey, strongSwan fires
the hook for many VTIs concurrently, so the writers lost-update each
other: an interface whose up-client add is overwritten is left admin-down
while its CHILD_SA stays installed.

Serialise all DB access by reusing vyos.utils.locking.Lock. A new
_vti_updown_db_lock() context manager wraps the three public context
managers, and remove_vti_updown_db() holds the lock across both the DB
processing and the os.unlink() to close the create/delete race.

Make the helpers absence-safe under the lock so callers no longer compose
a separate existence check with a locked operation:
open_vti_updown_db_readonly() yields None when the DB does not exist and
remove_vti_updown_db() is a no-op when it is absent. Drop the
now-redundant unlocked vti_updown_db_exists() pre-checks in vti.py and
vpn_ipsec.py and handle the None yield.

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