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/rtc_base/BUILD.gn b/rtc_base/BUILD.gn
index 1ee1ed5..d8af370 100644
--- a/rtc_base/BUILD.gn
+++ b/rtc_base/BUILD.gn
@@ -1156,6 +1156,8 @@
     "nattypes.h",
     "proxyserver.cc",
     "proxyserver.h",
+    "server_socket_adapters.cc",
+    "server_socket_adapters.h",
     "sigslottester.h",
     "sigslottester.h.pump",
     "socketstream.cc",