Configurable socket keep alive interval

[aksabg]

NetServer now supports tcpKeepAlive flag which is translated to Netty's SO_KEEPALIVE flag. However, the keepalive interval, idle time and count cannot be set through settings, which means it defaults to OS values (usually the idle time is 2 hours).

When using NetServer in conjuction with the AWS load balancer, the connections are closed when idle for more than 350 seconds.

The following Netty fields should be supported in NetServerOptions:

1. TCP_KEEPIDLE
2. TCP_KEEPINTVL
3. TCP_KEEPCNT

Reference: reactor/reactor-netty#2167

[aksabg]

@vietj I can add this feature, just need the permission to branch.

[vietj]

@aksabg please contribute it via a pull request

[aksabg]

@vietj I don't have permissions to create a remote branch required for a pull request.