add mod_tcp_keepalive to handle TCP-KeepAlive message sending strategy

[dblate]

Is your feature request related to a problem? Please describe. A clear and concise description of what the problem is. Ex. I'm always frustrated when [...] In some situation, like XiaoDu watch, which is sensitive to power consumption, may need to close the TCP-KeepAlive heartbeat message or increase the interval to send TCP-KeepAlive heartbeat message. Mod_tcp_keepalive can help handle situation like this.

Describe the solution you'd like A clear and concise description of what you want to happen. Mod_tco_keepalive support such params:

• Disable: close the TCP-KeepAlive heartbeat message sending strategy or not
• KeepIdle: period to send heartbeat message since there is no data transport in tcp connection
• KeepIntvl: period to send heartbeat message again when last message is not applied
• KeepCnt: count to resend heartbeat message when last message is not applied

Describe alternatives you've considered A clear and concise description of any alternative solutions or features you've considered.

Additional context Add any other context or screenshots about the feature request here.

[dblate]

pr: https://github.com/bfenetworks/bfe/pull/886 There are some issues in code analysis, should I fix them?