Refactor reading TIFF metadata (#82)

* Refactor reading TIFF metadata

* Fix Python bindings

* Fix clippy

* Don't use stateful cursor when reading tags

* Refactor `ImageFileDirectoryReader` to support reading individual tags

* Expose `next_ifd_offset`

* Don't make ImageFileDirectoryReader stateful

* Remove `get_metadata` from AsyncFileReader
Use file APIs directly

* Remove `get_metadata` impl from reader

* Rename to PrefetchBuffer

* Fix doctest

Author: kylebarron

Cargo.toml

@@ -33,6 +33,7 @@ tokio = { version = "1.9", features = [
     "rt-multi-thread",
     "io-util",
 ] }
+tokio-test = "0.4.4"
 
 [features]
 default = ["object_store", "reqwest"]