New OID database - with public definitions in oid_db.h. Removed old OID

New OID database - with public definitions in oid_db.h. Removed old OID
database including the head oid.h and definitions such as enum oid_value
and struct oident. The new OID database uses the same string names as
before but the 'protocol' is gone. There are now only two representations
raw OID (int *) and string. Functions with prefix yaz_string_to_oid
converts from string to OID; functions with prefix yaz_oid_to_string
converts the other way.