Represent sets and maps as subdirectories

This removes the s-expression encoding from the database

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
db_get_by_{uuid,name_label} are implemented in terms of read_records_where

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Add an index of reference to table name

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Implement get_table_from_ref and is_valid_ref

Signed-off-by: David Scott <dave.scott@eu.citrix.com>