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