Proposal: [MySQL] Add replication keep alive.

[Rentacookie]

When the source MySQL database is idle for long periods of time, write checkpoints in powersync will be sparse.

To get around this, we make regular updates to a powersync keepalive table on the source database. This then ensures regular commit events, and correspondingly regular write checkpoints.

If the PowerSync MySQL user has got the appropriate permissions, PowerSync can create and maintain the keepalive table.

If instead the MySQL user has more limited permissions, it would be up to the user to create the table themselves. The Powersync MySQL user would still need a minimal number of permissions on the table though for this to work:

CREATE TABLE IF NOT EXISTS
   <database>.powersync_keepalive(
	id INT PRIMARY KEY DEFAULT 1,
	last_ping TIMESTAMP(3) DEFAULT CURRENT_TIMESTAMP(3)
)
GRANT SELECT, INSERT, UPDATE ON <database>.powersync_keepalive TO 'repl_user'@'%';

If the table has not been pre-created and the PowerSync MySQL user does not have the required permissions, the keepalive mechanism is disabled and a warning is logged.

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