[Connection pool] skip heartbeat check in keep-alive mode

[datbth]

Summary

In connection pool, skip heartbeat check when keep-alive is enabled.

Description

• Before:
  • Every acquire() call on the connection pool runs a heartbeat query (given that testOnBorrow of the pool is kept as true)
    • One big purpose of a connection pool is to avoid the overhead of the initial connection step. However, this heartbeat query execution results in an equivalent or even higher overhead, especially when the heartbeat query fails and the pool has to make a new connection.
• After:
  • When clientSessionKeepAlive is true, skip the heartbeat query check and check only the client-side connection status.
    • NOTE: This behavior should be documented properly so that the user is mindful about configuring clientSessionKeepAlive and its related configs (clientSessionKeepAliveHeartbeatFrequency, SESSION_IDLE_TIMEOUT_MINS) to make the connection pooling more efficient.

Other changes

• Refactor heartbeatAsync into checkHeartbeat:
  • When the heartbeat query fails, resolve the returned Promise into false instead of raising an Error.
  • Reuse the function heartbeat to reduce code duplication and ensure the consistency of "heartbeat" queries.
• Check client-side connection status first (i.e. short-circuiting the logic) to avoid unnecessary heartbeat query executions.

[github-actions]

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[datbth]

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