Previous commit changed bincode deserializes in maildir and sqlite3.rs
from bincode::deserialize_from to using bincode::config::DefaultOptions
and bincode::Options trait's method deserialize_from.
However, these two different deserializes use a different default
settings: https://github.com/servo/bincode/issues/348
Specifically, varint encoding for integers is the default for
DefaultOptions but not when using bincode::{de,}serialize_* functions.
That means that serialized structs were not able to be deserialized.
This commit makes all {de,}serializations use the DefaultOptions
interface rather than the top level functions.