cassandra: Add option to select (hopefully) more efficient batch insertion algos

This adds a new config option for the cassandra backend, 'directory_entries_insert_algo', with three possible values:

• 'one-per-one' is the default, and preserves the current naive behavior
• 'concurrent' and 'batch' are attempts at being more efficient

Addresses the current performance bottleneck, according to swh/infra/sysadm-environment#3493 (closed)

Migrated from D6139 (view on Phabricator)