Fixed RD-15170: Reuse existing DASes when registering the same credential

[bgaidioz]

When a new DAS is registered, the cache introduced in that changeset, that indexes DAS instances by their options, is updated. If a redundant registration request is received for the same DAS type with identical options, the existing DAS instance is returned. This prevents the creation of a duplicate DAS and the associated synchronization with its external system.

The latest patch works as follows:

• an index from DAS ID to DAS config, which is used to makek sure explicit IDs are sent with a matching known config
• an index (using Cache/CacheLoader) that indexes runtime DASes from the config.

When a DAS is created, assuming the config matches a potential DAS ID passed among the parameters, the runtime DAS cache is accessed to either create a populate a new entry, or reuse a known one.

[bgaidioz]

During review, we note that after this patch, deleting and re-creating the exact same credential, will hit the cache and will call the getTableDescription of the formerly allocated DAS instance. It's up to the DAS to figure out whether it should update its list of tables or not (certain DASes would need to).