Don't clear caClientCallbackThreadId in CA's exit handler

documentation/RELEASE_NOTES.html

@@ -16,6 +16,15 @@
 
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+<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