Move ServerSocket code to separate files and into test target

Classes AsyncProxyServerSocket, AsyncSSLServerSocket, and
AsyncSSLServerSocket are used only by test and example code.

Moved to server_socket_adapters.{cc,h}, and to the
rtc_base_tests_utils build target.

In the process, also deleted a few ancient and unattributed TODO
comments.

Bug: webrtc:9798
Change-Id: I21279c92bd8f1354fab7eeaf1f9697fedfc760e1
Reviewed-on: https://webrtc-review.googlesource.com/c/107735
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26039}
diff --git a/p2p/BUILD.gn b/p2p/BUILD.gn
index a2aa5ff..7d26947 100644
--- a/p2p/BUILD.gn
+++ b/p2p/BUILD.gn
@@ -214,6 +214,7 @@
     ":rtc_p2p",
     "../rtc_base:checks",
     "../rtc_base:rtc_base",
+    "../rtc_base:rtc_base_tests_utils",
     "../rtc_base/third_party/sigslot",
     "//third_party/abseil-cpp/absl/memory",
   ]