v10.1.1: fix import/export operations on iOS

[JaffaKetchup]

• Converted import/export operations to use chunking instead of streaming
• Improved efficiency of import/export operations

Fixes #178.

[JaffaKetchup]

The problem is that using manual chunking seems to grow the database rapidly for some reason, and uses considerably more memory, whereas streaming seems to be slightly slower, but uses much less memory and does not appear to grow the database at all.

Unfortunately, the growth caused by the chunking is quite significant (it seems to be the entire amount of the import stores' tiles), and that might make this unsuitable for production, especially as it cannot be shrunk afterward.

EDIT: I've reduced the memory growth rate significantly by using more transactions - whilst this does mean potentially slightly reduced performance, it appears not to be noticable. This also resolved the size growth of the database file.