This puts caching in mentat_db, adds a reverse lookup capability for
unique attributes, and populates bidirectional caches with a single
SQL cursor walk.
Differentiate between begin_read and begin_uncached_read.
Note that we still allow toggling within InProgress, because there might be
transient local state that makes starting a new transaction impossible.
* Nit: Alphabetical ordering of imports
* Create Cache and provide functions for calling it
* Get tests working. Move to using NamespacedKeyword over KnownEntid in function signature
* Add is_cached check to caching tests
* Move lazy and add/remove boolean flags to enums
* Move function definitions into generic trait and implement trait for AttributeCache
* Remove lazy cache and generalize cache
* Update tests
* Eager cache becomes simple key value store. AttributeMap handles attribute storing specifics
* Update tests to test presence of correct values in cache
* Move EagerCache, AttributeValueProvider and ValueProvider into mentat_db
* Add test for get_for_entid
* Add test for lookup attribute
* Make caches cloneable. Add value_for alongside values_for
* Use cache in attribute lookups
* Split test for values and value and add cardinality
* address review feedback r=rnewman
