Implement Leapfrog Triejoin for worst-case optimal join in LMDB

[hmottestad]

Implement Leapfrog Triejoin for worst-case optimal join in LMDB.

Only supports joins with cycles and only support LMDB store where all 6 triple store indexes are in use (SPO, SOP, PSO, POS, OSP, OPS) so that we can use the indexes as tries.