Fix panic on sse-server shutdown

[hotafrika]

This is the fix for the "panic on sse-server shutdown when there is at least one active connection" issue.
It does the following:

1. after calling the Shutdown() function it doesn't allow new sse-connections to be established
2. it avoids sending to already closed removeClient channel

[insano10]

I've tested this forked version in an application utilising go-sse and can confirm it fixes the panic on shutdown

[insano10]

LGTM