debugger: count space for line numbers correctly
v8: ugprade to 3.20.17.14
zlib: fix write request reference counting

Keep track of the reference count, don't make the wrapper object weak
when there are pending write requests.  Fixes a regression from c79d516.
dtrace, mdb_v8: support more string, frame types

This change makes several improvements to the ustack helper and MDB
support:

- ustack helper and MDB: add support for two-byte strings
  (necessary to print many filenames in stacktraces in 0.10 and later).
- ustack helper: fix position numbers, which were off by a factor of two
- ustack helper: fix frames with undefined Scripts (e.g., "RegExp")
- ustack helper: add stub frames
- MDB: add support for sliced strings
- MDB: sync up with changes from the illumos version of the module

Fixes #6309
Closes #6318
mdb_v8: remove useless check
assert: indicate if exception message is generated

AssertionError.generatedMessage is now true when
AssertionError.message was generated from expected and actual

Fixes #5836, #6206
buffer: check data is not null

Because it's possible for the data within a Buffer instance to be
altered after instantiation, or in case a user attempts to do something
like the following:

Buffer.prototype.fill.call({}, 10, 0, 10);

It doesn't result in a segfault.
buffer: add buf.toArrayBuffer() API
lint: fix a cpplint error

Added a NOLINT so that cpplint won't complain about some code.
cares_wrap: remove superfluous 'this' keyword
cluster: fix premature 'disconnect' event

Don't emit the 'disconnect' event until all workers have gone away.
Before this commit, the event was emitted when all open handles were
closed, which usually - but not always - amounts to the same thing.

Fixes #6346.