Add SequentialDataBase as a better SimpleDatabase

[ZZZank]

This PR is made for resolving this issue:

but unlike PR128, who made a stupid and not-working LongSet, this PR replaced BitSet with IntOpenHashSet for id usage lookup, with a lowerBound for skipping ids that are certainly used when generating new id, it works like this:

• when generating new id through nextID, it will return the first id not in ids and in range [lowerBound, Integer.MAX_VALUE), if found, lowerBound will be set to id+1, and the id will be added to ids
• when removing id (removeID()), lowerBound can be set to the removed id because it might be the smallest unused id now. But note that this will not happen when removed id is smaller than lowerBound

[jchung01]

Isn't it better to just have QuestLine and QuestLineDatabase also use randomly-generated IDs as GTNH does? i.e. have them both extend RandomIndexDatabase instead of SimpleDatabase. There is no underlying idMap necessary for generating and tracking random IDs, which should resolve the shown issue due to questlines.

[ZZZank]

Isn't it better to just have QuestLine and QuestLineDatabase also use randomly-generated IDs as GTNH does?

yes it will be a better solution, but I think this should be in another PR, since this PR only focuses on improving SimpleDaraBase

[ZZZank]

since #130 provides a better fix for questline entries, this PR only focuses on providing a better SimpleDatabase now ¯\(ツ)/¯

I added a new database implementation SequentialDataBase, specialized in handling sequantial and dense ids. The SimpleDatabase is now just an "empty" subclass for SequentialDataBase, kept here for backward compat