i2ptunnel: HTTPResponseOutputStream cleanup

This is very old code; a lot more is possible but this is a start. Verified all main() tests pass.

• Reduce buffer size to 4K to match that used elsewhere in i2ptunnel
• Grow buffer by 2x instead of 1.5x
• Eliminate numerous dup method calls
• Eliminate write(byte[]) method present in super
• Rewrite headerReceived() to be more efficient
• Pre-convert constant Strings to bytes
• Remove _context field
• Remove isNL() method
• Null out _headerBuffer unconditionally