Use multiple keys once getKeys becomes available

[fregante]

Related:

• Use getKeys where available webext-storage-cache#50

webext-options-sync currently stores everything in a single item, this limits the total amount of data a single instance can store.

I can see this after only ~10KB:

By using getKeys I could also have a OptionsSync#get() method, not just OptionsSync#getAll()

It might be a good time to also change serialization method:

• Deserialize FormData sindresorhus/project-ideas#137