Fixes the following conversion problem for BlockedUser:
```
panic: interface conversion: interface {} is map[string]interface {},
not store.BlockedUser [recovered]
```
Resolves#1475.
* implement (strings) user details storage
* add rpc user details implementation
* return error from getUserDetail, rewrite tests to table tests
* make UserDetails store UserDetailEntry instead of strings
* update comment about user_details
* fix confusing return
* add user details support for memory store
* add engine.UserDetailEntry to service.UserMetaData
* add ListDetails support to memory storage
* add user details support to native migrator, ListDetails func to storage
* go mod tidy for memory storage
* increase memory storage test coverage, fix tests naming
* add ListDetails tests to memory storage
* add engine.ListDetails and service.[Set]Metas tests
* change Fprintf to Fprint (triggered by explicitly ignoring error)
* remove Delete from engine.UserDetail, implement list via same method
* adjust service.Metas to new engine.UserDetails signature
* introduce engine.UserDetail("all") consonant
* fix Meta user detail retrieval
* extend store implementations Delete method with UserDetail deletion
* make UserDetail test answer order-independent
* fix flaky test check in TestMemData_FlagListBlocked
* delete user details alongside with comments on deleteme request
* add tests to UserDetail store.Delete implementations
* clarify engine module user details consonants names
* update comments to reflect current state of code
* check for value absence instead of it's length
* revert unneeded code change
* add extensive commentary on UserDetail return type
* remove unused check condition
* clarify UserDetail tests to be truly stateless
* add clarifying comment for pre-table test
Dmitry VerkhoturovandUmputun