memory management on lattice traversal

[shahin]

Currently, all intermediate and temporary results continue to reside in memory until the program completes. This negates the benefit of depth-first search over the sequence lattice, which should require relatively little working memory (see Zaki 2001, section 4.3)

When searching the sequence lattice in enumerate_frequent_seq(), unnecessary data structures should be deleted, compressed, or flushed to disk as final output.