Fixed key (s, p, o) order for common value keys in the query planner.
Fixed two bugs in the SortJoinStream class.

The first was highlighted in levelgraph-jsonld and it created the
'homes in paris' test case. It was caused by a too early close of
the internal stream.
The second was caused by a wrong skipping of triples in the merge
sort implementation.
Removed setImmediate usage to work in the browsers.
Added a deprecation warning for join and joinStream.
Remove readStream after destroy because of limit in SortJoinStream
Stub instead of spy on console.warn for API deprecations.
Haunting spurious bug in browser.
Merge pull request #51 from mcollina/v0.7.1-wip

v0.7.1 wip