Streaming search

Created by: ijt

Sourcegraph search currently sends search results back all at once rather than streaming them back as they are found. A streaming solution would be more responsive, showing results as soon as possible, and also more reliable since it would no longer be necessary to set artificially short timeouts in order to make search responsive when the query spans multiple repositories.

For reference, here is an earlier version of this issue that was framed with too much focus on some technicalities.