Reduce database connnections required for routes::users::details #888

routes::users::detals() requires many DbConn. Its function signature requires a DbConn and three PlumeRockets, which has DbConn as a field. Additionally, other DbConns are retrieved in internal functions.

This occurs 503 error on proceeding, especially in the case of low database connection limit.

To fix this issue, we need to:

~~Extract DbConn from PlumeRocket as said at #797 and tried at #805~~ I gave up. See #889.
Change method signature of implementations of FromIds. ctx argument of from_id, from_activity and from_db should be a database connection instead of PlumeRocket (definition of FromId trait itself need not change)
Share database pool with SearchActor (or remove dependency on database connection from SearchActor)
Extract thread worker tasks in routes::users::details() as an actor's method, such that holds DbPool and retrieves DbConn on demand