Support custom timestamp, level, msg key names.

[dpc]

Timestamp could be really OwnedKeyValue with the value being a closure, with a possibility of an user adding it's own. The problem it's evaluation would happen a bit later (important for async drain).

Level & msg are kind of special, but maybe something similiar could be done for them.

[dpc]

Maybe support for closures Fn(&RecordInfo) -> Serialize would fix all of these and some part of #8. This way no special handling of timestamp, level, msg in the drains is required and drain or the user can register any custom closures under any custom name.

[dpc]

Done.