memdb: Use gc worker routines for garbage collection
memdb: Use gc worker routines for garbage collection

Spawn gc workers equal to the number of writers

Perf:
$ GOMAXPROCS=8 go test -test.run TestGCPerf
final_node_count = 8000, average_live_node_count = 28568, wait_time_for_collection = 16ms
PASS
ok  github.com/Pleaset3rm1n4l/memdb16.668s

t3rm1n4l/nitro,t3rm1n4l/nitro,t3rm1n4l/nitro