New batch of HTTP/2 protocol fixes

[marty1885]

New fixes to HTTP/2 implementation

Bug fixes & performance:

• Corrected padding format when sending frames
• Fix overly strict asserts in debug mode
• Proper support for tailers
• Remove limit of not supporting large headers + no body (treated as tailers)
• Greatly increase large file download speed
  • Increased RX window increment by 4MB instead of 65K
  • Increased RX window increment threshold from 16K to 256K
  • Increased our advertised initial window size to 512K over the RFC default of 65K
• Logs protocol error
• Optimize stream tracking when body is larger then available TX window
• Handle malicious servers intentionally triggering overflow in our TX window size
• Added support to send ping periodically
  • Disabled by default. Can be enabled with client->setPingInterval(seconds)
• Increase max allowed encoded header size from 2K to 2MB
• Avoid UAF on GOAWAY handling
• Relax some recoverable error to stream level instead of killing the entire connection

RFC Compliance:

• Ignore MSB for GoAway frame
• Window size increment of 0 is now an error
• The 1st INITIAL_WINDOW_SIZE settings affect all streams
  • Handles negative window correctly
• Respect server's DYNAMIC_TABLE_SIZE settings
• Headers containing upper case letter treated as error
• Sends our own RST_STREAM if we detect a recoverable problem instead of GOAWAY

Stress testing passed:

• Download Arch Linux ISO from Fastly at 78MB/s (near line rate from home, acceptable given RTT)

Changes:

• Environment variable DROGON_DISABLE_HTTP2_CLIENT=1 will disable HTTP/2 in client (escape hatch in case there's trouble)