nghttpx: Adjust TLS record size dynamically

Use the same behaviour the current Google server does: start with 1300
TLS record size and after transmitting 1MiB, change record size to
16384.  After 1 second idle time, reset to 1300.  Only applies to
HTTP/2 and SPDY upstream connections.
Add X-Next-Protocol header to backend request
nghttpx: Apply TLS record length limit to DATA frame payload

This is not obvious but it makes intermediaries flush and forward DATA
frame boundary without excessive buffering.  Since we have different
TCP connections frontend and backend, this may not work.  This is
still experimental.
Add LOG_NOTICE level logging for application lifecycle events
Fix heap-use-after-free due to duplicated push of DATA item
h2load: Make shutdown sequence simpler
Merge branch 'log_notice' of https://github.com/LPardue/nghttp2 into LPardue-log_notice
Merge branch 'LPardue-log_notice'
Fix bug that priority is ignored if dependency parent does not exist
Don't reprioritize stream if it is not in dependency tree

Normally stream is in dependency tree, but sometimes it isn't.  For
example, client does not put pushed reserved stream in dependency
tree.
nghttpx: Fix -L option help message

WARNING should be WARN.
nghttpx: Replace WARNING with WARN for consistency
Assign default priority if dependency parent stream does not exist
Add test to make sure that default priority is assigned
Assign default priority if dep_stream in PRIORITY does not exist
Fix C++ lib sample
h2load: Print used SSL/TLS cipher name and parameters
nghttpx: Use exit instead of abort for DIE macro
Bump up version number to 0.6.6, LT revision to 7:1:2
Update man pages