Use cznic/b B+tree implementation in place of GoLLRB for memstore

The benchmarks show significant wins here.

	$ benchcmp gollrb.bench b-gen.bench 
	benchmark                                   old ns/op       new ns/op       delta
	BenchmarkNamePredicate                      1631932         1409531         -13.63%
	BenchmarkLargeSetsNoIntersection            190792654       63748682        -66.59%
	BenchmarkVeryLargeSetsSmallIntersection     896154437       373475843       -58.32%
	BenchmarkHelplessContainsChecker            20719182678     14078301640     -32.05%
	BenchmarkNetAndSpeed                        32519019        20188665        -37.92%
	BenchmarkKeanuAndNet                        18319247        15224988        -16.89%
	BenchmarkKeanuAndSpeed                      30849568        18744134        -39.24%
	BenchmarkKeanuOther                         105552525       107620648       +1.96%
	BenchmarkKeanuBullockOther                  295395338       115193002       -61.00%

	benchmark                                   old allocs     new allocs     delta
	BenchmarkNamePredicate                      1339           1341           +0.15%
	BenchmarkLargeSetsNoIntersection            22585          23632          +4.64%
	BenchmarkVeryLargeSetsSmallIntersection     65776          69396          +5.50%
	BenchmarkHelplessContainsChecker            1713541        2036316        +18.84%
	BenchmarkNetAndSpeed                        17104          17240          +0.80%
	BenchmarkKeanuAndNet                        15816          15855          +0.25%
	BenchmarkKeanuAndSpeed                      16368          16493          +0.76%
	BenchmarkKeanuOther                         30134          30634          +1.66%
	BenchmarkKeanuBullockOther                  35510          36454          +2.66%

	benchmark                                   old bytes     new bytes     delta
	BenchmarkNamePredicate                      96162         96294         +0.14%
	BenchmarkLargeSetsNoIntersection            1172356       1249872       +6.61%
	BenchmarkVeryLargeSetsSmallIntersection     2810080       2992409       +6.49%
	BenchmarkHelplessContainsChecker            89233264      104999088     +17.67%
	BenchmarkNetAndSpeed                        1388793       1428110       +2.83%
	BenchmarkKeanuAndNet                        1263145       1250079       -1.03%
	BenchmarkKeanuAndSpeed                      1246956       1281546       +2.77%
	BenchmarkKeanuOther                         2021312       2024727       +0.17%
	BenchmarkKeanuBullockOther                  2671448       2742968       +2.68%