A few fixes:

• Fix a few nullability annotations https://github.com/ccgus/fmdb/commit/a76844e31768b9b76f2c5233d7ac38e12db4c7e1
• Add support for immediate transactions https://github.com/ccgus/fmdb/commit/b4bd097249685da62b6a627bc3286273dbf9a284
• Add explicit support for exclusive transactions, too https://github.com/ccgus/fmdb/commit/cb9284507706217deddbd5e148674ac9beeac793