Merge r1679032, r1679192, and r1680276 from trunk:

r1679032:

mod_ssl OCSP Stapling: Don't block initial handshakes while refreshing
the OCSP response for a different certificate.  mod_ssl has an additional
global mutex, "ssl-stapling-refresh".

Not mentioned in CHANGES:

Stapling no longer uses a mutex when using a stapling cache
implementation which doesn't require it.  (A further, unrelated
code change to mod_ssl is required to allow the use of memcache 
as a stapling cache, and I haven't tested with distcache; thus
it isn't clear if this helps in practice yet.)

r1679192:

Fix regression in check for cached response
(Essentially) Submitted by: ylavic

r1680276:

OCSP stapling: slight simplification to some internal interfaces,
add a few comments and sanity checks

Submitted by: trawick (with assist from ylavic)
Reviewed by: jim, jorton


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1681320 13f79535-47bb-0310-9956-ffa450edef68