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

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.