fix: Accept() must not close the listener socket on success

[cdgraff]

Summary

NodeSRT::Accept() (src/node-srt.cc) called srt_close() on the listener socket on the success path, so the listener was destroyed after every accepted connection.

In libsrt, srt_accept() returns the newly accepted socket and leaves the listener open so it can keep accepting - this diverged from that contract:

• A server could accept only one connection per listener; after the first accept() the listener went BROKEN then NONEXIST, and any further caller timed out.
• The usual workaround, re-creating (bind+listen) the listener after every accept, drops its backlog of pending handshakes. Under a burst of simultaneous callers this meant only one connection got through and the rest failed.

Fix

Removes the success-path srt_close(socketValue) (and the following no-op reassignment), so Accept() only returns the accepted fd and leaves the listener open, matching libsrt semantics. The caller is responsible for closing the listener when done. The failure path is unchanged.

Test

Adds spec/accept_keeps_listener_open_spec.js, a regression test covering both the sync (SRT) and async (AsyncSRT) accept paths: it accepts several connections in a row from the same listener and asserts the listener stays LISTENING throughout.

Verified locally:

• Reverting just the src/node-srt.cc change and rebuilding the addon reproduces the bug: the listener's state goes BROKEN right after the first accept, and the next connection attempt errors out (invalid listener socket ID value).
• With the fix applied, the full spec suite passes (23/23), including the new test.

Fixes #79

cc @birme