Support more than 32bit ApproximateSize bindings

[larskuhtz]

Currently the binding for approximateSize stores the result as uint32 (

leveldown/binding.cc

Line 1036 in e1c3463

napi_create_uint32(env_, (uint32_t)size_, &argv[1]);

). This supports only databases of up to 4GB and returns wrong results for larger DBs.

It would be nice if approximateSize could either

1. support all of Number.isSafeInteger(),
2. support larger numbers by representing them as nearest float (it's "approximate" after all),
3. return BitInt, or
4. at least return an error if an invalid result is returned.

Not sure, which of the options would be best.

[vweevers]

I prefer option 1. Brings us into petabyte or exabyte territory, without a breaking change.

[larskuhtz]

With option (1.), how should the case be handled when then database returns a value that is to big? Throw an error, return -1, or just return whatever napi_create_int64 gives (essentially using option 2. as fallback)?

Currently, it just overflows, which isn't ideal.

[vweevers]

With option (1.), how should the case be handled when then database returns a value that is to big? Throw an error, return -1, or just return whatever napi_create_int64 gives (essentially using option 2. as fallback)?

Return whatever napi_create_int64 gives. Because as you pointed out, the db size already is approximate. And if you have a database that big, I'm not sure LevelDB is the right fit anyway.

[larskuhtz]

cf. PR #777