[Minor] Do not fetch a hyperscan database that is already installed

[Minor] Do not fetch a hyperscan database that is already installed

A map is put in the compilation queue by its read callback and announced by a
notification once the database is there, and both paths load it, so every map
update cost twice the traffic: megabytes for a map of a few thousand patterns.

Skip the load when the helper already has a database. That cannot keep a stale
one: every read allocates a helper of its own with no database at all, its
patterns never change afterwards, and a database only ever gets installed for
the content of that very helper, the load verifying the digest before it does
so.