Support multiple 'closest' LSIF uploads in GraphQL

Created by: efritz

https://github.com/sourcegraph/sourcegraph/pull/8269 made it possible for there to be many 'closest' uploads for a code intelligence query. This changes returns all of them from lsif-server (instead of just arbitrarily returning the first one), and queries each of them in the GraphQL code intel resolvers.

Definitions and hovers will query each dump sequentially and the one with the first non-empty result will be returned without querying the remaining dumps. All dumps will be queried for reference results and the results will be appended together.