Storage: Use a more descriptive WAN cache keyclass in 'SqlBlobStore'

Storage: Use a more descriptive WAN cache keyclass in 'SqlBlobStore'

In order to make statistics from WANObjectCache more useful,
each kind of key should generally have a unique first make*Key
component. This usually corresponds 1:1 to a call to make*Key,
unless the same key format is duplicated in muliple places.

In this case, change ('BlobStore', 'address', …, …)
to ('SqlBlobStore-blob', …, …).

Bug: T197849
Change-Id: Iba3ae1bf947d6f56c72df2cf3f2160d458d38c66