Refactoring around Field (#673)

* Refactoring around Field

Removing the contract about the order of the field, and the
field id allocation.

* Update delete_queue.rs

* Update field.rs

tests/failpoints/mod.rs

@@ -1,5 +1,4 @@
 use fail;
-use std::io::Write;
 use std::path::Path;
 use tantivy::directory::{Directory, ManagedDirectory, RAMDirectory, TerminatingWrite};
 use tantivy::doc;