[FEATURE, INTERNAL] Adds a new iterator for journaled strings to improve runtime

Additional iterator uses only one segment iterator in case if host and insertion buffer are initialized with the same specialization.
[API] Removes the implementation of the unbalanced tree for journaled sequences.
[INTERNAL] Improves iteration over journaled string.
Merge pull request #157 from rrahn/develop

[FEATURE, INTERNAL] Adds a new iterator for journaled strings to improve runtime