Add metadata method to public API

[RobDickinson]

Currently KVTree has GetPath and GetSize methods to return metadata about the datastore, but this pattern will pollute the API if we add a bunch of these down the road. Instead let's replace those current methods with a single Metadata method that populates a struct with all those details.

struct KVTreeMetadata {                              // add
    string path;
    size_t size;
    size_t leaves;
    size_t nodes;
}
class KVTree {
  public:
    const string& GetPath();                         // remove
    const size_t GetSize();                          // remove
    void Metadata(const KVTreeMetadata& metadata);   // add

This will be very helpful for changes like #10 that affect internal structure and are easier to validate if more internal state can be exposed easily.

[RobDickinson]

Existing tests that check path/size were changed to this pattern:

    KVTreeMetadata kvmeta = {};
    kv->Metadata(kvmeta);
    ASSERT_TRUE(kvmeta.path == PATH);
    ASSERT_TRUE(kvmeta.size == PMEMOBJ_MIN_POOL);

[RobDickinson]

Just a little refactoring here at the end -- shouldn't be using name and path interchangeably anywhere now. (should just be path)

[RobDickinson]

No additional unit tests were added for this change -- just being able to execute all current tests is good enough.