Indexing: configure CPU count through index options (#704)
This change allows the shard concurrency to be set through index options. This
is much more convenient than the current way to limit CPU through the server
flag `-cpu_fraction`.
In a follow-up we'll expose shard concurrency through Sourcegraph site config,
and pass it through these index options. Maybe we can deprecate and move away
from `-cpu_fraction` in favor of this approach.