Refactor and add unit tests for roomserver/query/query.go

[matrixbot]

This issue was originally created by @kegsay at matrix-org/dendrite#929.

to make it much easier to see what is going on

[matrixbot]

This comment was originally posted by @kegsay at matrix-org/dendrite#929 (comment).

• the database methods are all over the place and to pull out basic info requires 3-4 calls
• the prolific use of "here's a pointer to a response, write whatever you want as a side-effect when I call you!"
• the constant manual type-casting to get []Event into anywhere, roomserver has it's own types.Event :/
• the lack of any unit tests beyond getAuthChain
• the database interface is so huge that it makes it difficult to see exactly what each query impl actually needs from the database, sometimes it's very little