Fix bug in priority tree

This change fixes the bug that stream is out of dependency tree if the
number of nodes in a dependency tree which we add new node to is
already maximum (NGHTTP2_MAX_DEP_TREE_LENGTH) and the number of
maximum concurrent streams is more than more than
NGHTTP2_MAX_DEP_TREE_LENGTH.
nghttp: Allow multiple -v option to increase verbosity

Now the number of -v option specifies verbosity level.  Current all
verbose output are turned on for at lest one -v option, except for the
debug output for each data chunk which is only turned on more than one
-v options.
src: Disable SSL_MODE_ENABLE_PARTIAL_WRITE for apps which use libevent
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.
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