Produce a unique namespace at compile time

[vweevers]

So that symbols don't clash when multiple versions of the addon are loaded into node.

Closes #686. Summary of that thread: when two 5.x versions of leveldown are loaded side by side, for example 5.3.0 and 5.0.2, their (static) methods conflict. E.g. if you open a 5.0.2 db, on completion Node.js will call the BaseWorker::Complete method of 5.3.0.

Some remaining questions:

• Could this be considered a bug in node?
• Is there a better solution?
• Only makes leveldown symbols unique, not leveldb symbols
• I've only tested debug builds so far

[ralphtheninja]

Could this be considered a bug in node?

It seems to me that any native module using the same technique should suffer from same problems.

Only makes leveldown symbols unique, not leveldb symbols

I think this should be fine, since we link statically with leveldb.

[vweevers]

I'll leave this hanging for a few days, in case someone has a better solution, or additional insights.

[vweevers]

@ralphtheninja the leveldb symbols are still exported though, which might cause problems? Similar to how Node.js exporting OpenSSL symbols blocks native addons from using a custom OpenSSL version (nodejs/help#1724).

I'm reading about hiding symbols (-fvisibility=hidden), might help.

[vweevers]

I came across nodejs/node-addon-api#460. These folks recommend always using -fvisibility=hidden for mac. Lemme ask why! It seems to stem from nodejs/node-addon-api#456, which describes a very similar issue 😃

[vweevers]

Closing in favor of #688.

Don't delete this branch yet because that would break the repro which I'd like to keep around at least until I've tested the prebuilt binaries of the next release.