Low memory vtile decoding / replace google libprotobuf with protozero

[springmeyer]

Implements plan described in #422

Key context: #474

Before merging:

• start manually integrating some of Implement gzip compression in the C++ layer #456 into this branch
• test/branch some modules depending on vtile like tilelive-bridge and tilelive-vector (will require usage updates to avoid vt.parse())
• improve code coverage

[joto]

The code quite often has the pattern:

protozero::pbf_reader message(...);
while (message.next()) {
    if (message.tag() == N) {
        ...
    } else {
        message.skip();
    }
}

This should be replaced by the equivalent, but much simpler:

protozero::pbf_reader message(...);
while (message.next( N )) {
    ....
}

Also, for convenience, pbf_reader can be instantiated with a std::string. pbf_reader message(str.c_str(), str.size()); is the same as the shorter pbf_reader message(str);.

This overload of the constructor is also sometimes useful: pbf_reader(std::pair<const char *, size_t> data) noexcept;

Looking at the pbf_get_layer function I am thinking it would be a lot cleaner if it could just return the message. We could enable this by giving pbf_reader an operator bool() which would basically do the same thing as next(). Does this make sense?

[springmeyer]

thanks @joto - made improvements in a013732

[springmeyer]

Okay, we're now 100% (https://coveralls.io/builds/3518078), so this is ready to merge.