When the cache is constructed with expiry == 0 the underlying storage is
never instantiated and get_ptr() asserts via caching_enabled(). This is
fine for callers that need a handle into the cache, but it makes get_ptr()
unusable for write-only insertions on caches whose expiry is configurable
at runtime (e.g. caches driven by a LiveUpdate config option that the
operator may set to 0).
Add a new insert(k, load) method on loading_cache that returns a future<>
and is a no-op when caching is disabled, otherwise forwards to
get_ptr(k, load) and discards the resulting handle. This completes the
disabled-mode safety contract of the cache for the write side, mirroring
the fallback that get() already provides for the read side.
Switch authorized_prepared_statements_cache::insert() from
get_ptr().discard_result() to the new insert(), which fixes the crash
'Assertion caching_enabled() failed' in
authorized_prepared_statements_cache::insert() that occurs when
permissions_validity_in_ms is set to 0 and a prepared statement is
executed under authentication.
Fixes SCYLLADB-1699
918130befd