Make search sync asynchronous when merging orgs

[sausage-todd]

There are two issues with search sync being synchronous right now:

1. It might take longer to sync to search than to execute a finalizing
   temporal workflow. If this happens, temporal workflow will notify
   frontend about finishing org merging before this method returns. This
   means, frontend learns about org merging being done before it learns
   the org merging even started.
2. Synchronous search sync might fail. Since we execute it outside of a
   database transaction, the error frontend sees will be misleading.
   Because, on one hand, the request to merge orgs failed, but on the
   other hand it actually did something, transaction got commited, it
   can't be retried.

As a result, the solution:

1. Make search sync asynchronous, so it doesn't fail (or fails much less
   often)
2. Execute it before starting executing temporal workflow. So there is
   as little delay as possible between starting a temporal workflow and
   returning something to frontend