[v6.x backport] src: simplify memory management using `node::Malloc()` and friends
https://github.com/nodejs/node/pull/8482
Without #8482, it's impossible to back-port pull requests that use the new APIs.
cc @addaleax - this is your PR plus some minor tweaks:
- Some updates to cares_wrap.cc for code that doesn't exist in v8.x
-
ArrayBufferAllocator::Allocate()
is different in v6.x because of #7082 - I removed
LowMemoryNotification()
in test_util.cc because of double symbol conflicts