Implement a new multi-bucket method for vshard

RFC: https://docs.google.com/document/d/1cSuKG2rI7ydehSjsnmATTEJ3i_d2WOIryVc7j7jw3cc

Algorithm:

1. Group buckets based on router bucket cache and replicate sets.

2. For each replicate set:

   a. Transmit presumed replicate-set-affiliated buckets and a function with arguments to the replicate set.

   b. Reference the transmitted buckets. If any buckets are missing, remember them.

   c. Execute the function and save the result.

3. Return results from 2.c and unfound buckets from 2.b to the router.

4. Collect results from all replicate sets:

   a. If there are no unfound buckets, return the result as success.

   b. If there were unfound buckets in step 3, search for them and update the router bucket cache through bucket_resolve.

5. For these buckets, go to step 1.

Remember to handle errors and timeouts.