dns, cares: don't filter NOTIMP, REFUSED, SERVFAIL

Report the aforementioned DNS server failure codes as-is, stop c-ares
from reporting them as ARES_ECONNREFUSED.

Reported by Steve Freegard.
gitignore: Ignore release tarballs and shasum files
buffer: optimize Buffer.prototype.toString('hex')

Move the implementation to C++ land. The old JS implementation used
string concatenation, was dog slow and consumed copious amounts of
memory for large buffers. Example:

  var buf = Buffer(0x1000000);  // 16 MB
  buf.toString('hex')           // Used 3+ GB of memory.

The new implementation operates in O(n) time and space.

Fixes #4700.
debugger: Make the debugger timeout configurable

If the NODE_DEBUGGER_TIMEOUT environment variable is set, then use
that as the number of ms to wait for the debugger to start.

This is primarily to work around a race condition that almost never
happens in real usage with the debugger, but happens EVERY FRACKING
TIME when the debugger tests run as part of 'make test'.
buffer: optimize Buffer.prototype.write(s, 'hex')

Move the implementation to C++ land. This is similar to commit 3f65916
but this time for the write() function and the Buffer(s, 'hex')
constructor.

Speeds up the benchmark below about 24x (2.6s vs 1:02m).

  var s = 'f';
  for (var i = 0; i < 26; ++i) s += s;  // 64 MB
  Buffer(s, 'hex');
speed up net write if socket buffer large enough

add uv_try_write() to libuv, try to write as many bytes as possible
into socket buffer for nonblocking socket, if all data is written
successfully, async uv write can be avoid, thereby saving the
consumption of uv context and queue operations.