api/sync: allow to configure requests for pool size and retries

by default:
- increase the connection pool size and
- set retry to 3 (number of retries each connection should attempt).

These config params can be set directly from config files for 'remote'
api clients. For example, to specify these in an objstorage client
config file:

  objstorage:
    cls: remote
    args:
      url: swh-objstorage:5003
      max_retries: 5
      pool_connections: 100