Handle database connection within partition strategies

Problem

Partition strategies use Gitlab::Database::PostgresPartition to get the partition details.

These partition info is in-turn used from different places, outside the strategy (eg: models). If a proper connection is not used explicitly then the strategy might result in unexpected results, eg: using the default main database for ci/sec tables. Such cases for tough to debug and have a greater negative impact.

Solution

Always use the proper connection while accessing the partition related module (eg: strategies). If the connection is not specified explicitly, then the partition strategy should fallback to its model's (this is passed while instantiating the strategy) connection.

References

• #340976
• !201924 (comment 2696426276)
• !202033 (merged)