[JENKINS-27555]: Re-implements JNA agent with non-blocking code

[ydubreuil]

Fix JENKINS-27555.

This PR re-implements JNA agent with a non blocking socket acceptor to fix socket leak caused by blocking on accept() native call. Current implementation leaks a socket and a thread for every build.

Leaked threads have this stack:

"Thread-7" prio=10 tid=0x00007f6d30113000 nid=0x1f34 runnable [0x00007f6d54625000]
   java.lang.Thread.State: RUNNABLE
        at com.kenai.jffi.Foreign.invokeN3O2(Native Method)
        at com.kenai.jffi.Invoker.invokeN3(Invoker.java:1092)
        at jnr.unixsocket.Native$LibC$jnr$ffi$0.accept(Unknown Source)
        at jnr.unixsocket.Native.accept(Native.java:91)
        at jnr.unixsocket.UnixServerSocketChannel.accept(UnixServerSocketChannel.java:56)
        at com.cloudbees.jenkins.plugins.sshagent.jna.AgentServer$1.run(AgentServer.java:77)

[jenkinsadmin]

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[ydubreuil]

@reviewbybees

[stephenc]

👍 very good catch

[stephenc]

As plugin maintainer, I am merging this now