Don't clear caClientCallbackThreadId in CA's exit handler

documentation/RELEASE_NOTES.html

@@ -16,6 +16,15 @@
 
 <!-- Insert new items immediately below here ... -->
 
+<h3>Cleaning up with Multiple CA contexts in a Process</h3>
+
+<p>Bruno Martins reported a problem with the CA client library at shutdown in a
+process that uses multiple CA client contexts. The first context that triggers
+the CA client exit handler prevents any others from being able to clean up
+because it resets the ID of an internal epicsThreadPrivate variable which is
+shared by all clients. This action has been removed from the client library,
+which makes cleanup of clients like this possible.</p>
+
 <h3>Perl CA bindings fixed for macOS Mojave</h3>
 
 <p>Apple removed some Perl header files from macOS Mojave that were available

src/ca/client/ca_client_context.cpp

@@ -48,7 +48,6 @@ const unsigned ca_client_context :: flushBlockThreshold = 0x58000;
 extern "C" void cacExitHandler ( void *)
 {
     epicsThreadPrivateDelete ( caClientCallbackThreadId );
-    caClientCallbackThreadId = 0;
     delete ca_client_context::pDefaultServiceInstallMutex;
 }