Nodejs 12 support

[chrisdoc]

This PR aims to make node-dtrace-provider compatible with changes in v8 and allows the usage of the library with Node.JS 12. I ran the tests both for master with node 10.15.2 and on the branch with node 12.5.0 and on both I had 670 passing tests out of 699 total tests. I tested on MacOS 10.14.5 and disable SIP by executing csrutil disable in recovery mode. @chrisa do you have a continuous integration setup where I could test this PR.

[melloc]

Hi @chrisdoc ,

There's no CI setup currently. I have a FreeBSD, SmartOS and OS X box that I currently run the tests on. I'll test these changes on them for you this week, and put the results here.

[chrisdoc]

Thank you @melloc for your help in getting this tested

[melloc]

I tested the following combinations:

• SmartOS (joyent_20180813T212436Z), node v0.10.48, v0.12.16, v4.8.1, v6.17.0, and v8.15.1 (tested 32-bit and 64-bit for all versions)
• SmartOS (joyent_20190523T082605Z), node v10.14.2 and v12.7.0 (64-bit)
• FreeBSD 10, node v6.9.1 (64-bit)
• FreeBSD 11, node v9.3.0 (64-bit)
• macOS (version 10.14.6, Xcode 9.4), node v0.10.48 (32-bit and 64-bit), v4.9.1, v8.16.0, v10.16.0, v12.7.0 (64-it)

dtrace-provider failed to compile with node v0.12.18 on macOS due to nodejs/nan#676, which I can't really do anything about in this library. That's probably fine, since I imagine most people are using newer node versions.

I also had to make some minor tweaks to get the tests running on macOS, due to what seems to be a regression in using -Z to attach to USDT probes that haven't yet been defined. I'll follow up with a separate commit to update the tests to work once I'm convinced that's the problem, and that it's not something that libusdt should be updated to account.

[melloc]

Merged as 3cbb3c8. I'll prepare a 0.8.8 release once I figure out whether anything further needs to be done about the macOS issue.

[chrisdoc]

Thank you @melloc