ca: add sqlite index on RealisationsRefs(realisationReference)

Without the change any CA deletion triggers linear scan on large
RealisationsRefs table:

    sqlite>.eqp full
    sqlite> delete from RealisationsRefs where realisationReference IN ( select id from Realisations where outputPath = 1234567890 );
    QUERY PLAN
    |--SCAN RealisationsRefs
    `--LIST SUBQUERY 1
       `--SEARCH Realisations USING COVERING INDEX IndexRealisationsRefsOnOutputPath (outputPath=?)

With the change it gets turned into a lookup:

    sqlite> CREATE INDEX IndexRealisationsRefsRealisationReference on RealisationsRefs(realisationReference);
    sqlite> delete from RealisationsRefs where realisationReference IN ( select id from Realisations where outputPath = 1234567890 );
    QUERY PLAN
    |--SEARCH RealisationsRefs USING INDEX IndexRealisationsRefsRealisationReference (realisationReference=?)
    `--LIST SUBQUERY 1
       `--SEARCH Realisations USING COVERING INDEX IndexRealisationsRefsOnOutputPath (outputPath=?)

src/libstore/ca-specific-schema.sql

@@ -32,6 +32,8 @@ create table if not exists RealisationsRefs (
     foreign key (referrer) references Realisations(id) on delete cascade,
     foreign key (realisationReference) references Realisations(id) on delete restrict
 );
+-- used by deletion trigger
+create index if not exists IndexRealisationsRefsRealisationReference on RealisationsRefs(realisationReference);
 
 -- used by QueryRealisationReferences
 create index if not exists IndexRealisationsRefs on RealisationsRefs(referrer);

src/libstore/local-store.cc

@@ -152,6 +152,8 @@ void migrateCASchema(SQLite& db, Path schemaPath, AutoCloseFD& lockFd)
                     select id from Realisations where outputPath = old.id
                     );
                 end;
+                -- used by deletion trigger
+                create index if not exists IndexRealisationsRefsRealisationReference on RealisationsRefs(realisationReference);
             )");
             txn.commit();
         }