MINOR: ssl/cli: flush the server session cache upon 'commit ssl cert'

Flush the SSL session cache when updating a certificate which is used on a
server line. This prevent connections to be established with a cached
session which was using the previous SSL_CTX.

This patch also replace the ha_barrier with a thread_isolate() since there
are more operations to do. The reg-test was also updated to remove the
'no-ssl-reuse' keyword which is now uneeded.
diff --git a/reg-tests/ssl/set_ssl_server_cert.vtc b/reg-tests/ssl/set_ssl_server_cert.vtc
index 412e9f0..ccf7887 100644
--- a/reg-tests/ssl/set_ssl_server_cert.vtc
+++ b/reg-tests/ssl/set_ssl_server_cert.vtc
@@ -34,7 +34,7 @@
     listen clear-lst
         bind "fd@${clearlst}"
         retries 0 # 2nd SSL connection must fail so skip the retry
-        server s1 "${tmpdir}/ssl.sock" ssl verify none crt ${testdir}/client1.pem no-ssl-reuse
+        server s1 "${tmpdir}/ssl.sock" ssl verify none crt ${testdir}/client1.pem
 
     listen ssl-lst
         # crt: certificate of the server