Tweak SocketManager's port on Windows

[ashie]

SocketManger on Windows uses TCP to communicate with workers and it uses 10000 port by default. But this port is known as NDMP(Network Data Management Protocol): https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.xhtml?search=10000 and ServerEngine doesn't provide a way to change it manually.

To resolve the conflict this PR includes following two fixes:

• Use dynamic ports according to OS's setting
• Enable to change the port by environment variable SERVERENGINE_SOCKETMANAGER_PORT

[repeatedly]

Could you consider update appveyor setting to run tests on newer rubies or migrating to github actions after merged this patch?

[ashie]

Could you consider update appveyor setting to run tests on newer rubies or migrating to github actions after merged this patch?

Of course, I'll do it.

[repeatedly]

Merged!

[ashie]

Thanks!