Optimizations for immediate write code paths

Release 1.17.15

- [BUGFIX] http_client: make sure only one read per on_read() callback
  is performed in the header conversion bypass (-B) mode.
- http_client: with -E, assign random priority when stream is created.
- [OPTIMIZATION] On immediate write, place an ACK frame into the first
  buffered packet if an ACK is queued.  This reduces the number of
  standalone ACK packets.
- [OPTIMIZATION] Allow placing more than one STREAM frame from the same
  stream into an outgoing packet.  This change minimizes the number of
  buffered packets required to store several small HTTP messages by
  virtue of allowing more than one STREAM frame from HEADERS stream in
  the same packet.
- [OPTIMIZATION] Flush headers when writing to buffered packets.  This
  causes the headers to be written to the same buffered packet queue,
  thereby improving packet utilization, especially for small HTTP
  messages.

Client fixes and optimizations

Release 1.17.14

- [FEATURE] http_client can now collect stats: time to connect, TTFB,
  time to download, requests per seconds, and bandwidth.  See -t flag.

- [BUGFIX] http_client: -B, -K, and -r can be used at the same time.

- [BUGFIX] http_client: do not display cert chain before each request.
  Now this is only done once per connection, if the handshake is
  successful and -a option is given.

- [BUGFIX] Do not wait to dispatch on_close() after reading.  If a
  stream is scheduled to be closed after on_read() callback returns,
  close it immediately instead of waiting until the end of the tick.
  If client creates new request from on_close() event, they will be
  processed in the same tick instead of waiting for the next one.

- [BUGFIX] HEADERS stream is critical: always treat it with highest
  priority.

Sample program can send parallel requests in a single connection

1.17.12

Update header file and changelog: release 1.17.12

FIx strict aliasing when compiling with optimizations

Release 1.17.11

Fix strict aliasing warning in when compiling with optimizations

Fix parallel connections in the sample client code

Release 1.17.10

Fix the example program to be able to use parallel connections
again.  (See the -n argument.)

[BUGFIX] Engine: reduce minumum batch size from 256 to 4

Release 1.17.9

[BUGFIX] Engine: reduce minumum batch size from 256 to 4

[BUGFIX] Compilation issues on FreeBSD and 32-bit Linux

Release 1.17.8

[BUGFIX] Fix compilation on FreeBSD and 32-bit Linux

[BUGFIX] Enable PINGs to be sent again

Release 1.17.7

[BUGFIX] Do not unset PING alarm before ringing expired alarms.  This
prevented PING from ever being sent.

Fix a few corner cases

• Add failsafe: resume sending packets after some time
• [BUGFIX] Handle corner cases in send controller when packets are a) delayed or b) dropped during repackaging.
• [BUGFIX] Memory leak: destroy buffered packets during controller cleanup.

Do not send STOP_WAITING frames in Q044

Release 1.17.3

[BUGFIX] Do not send STOP_WAITING frames when using Q044