Image interface changes

[paskal]

Multiple preparations before implementing #572 which I don't want to mess up into image RPC implementation PR.

• sort imports in memory_store for consistency
• add a missing copyright notice to rpc.go file
• make all image.Store interface functions public: otherwise, it's impossible to use this interface from another package:
  
• make image.Store.Load return []byte instead of io.ReadCloser
• regenerate mock files in remark42 backend (for image and engine)
• go mod tidy in memory_store changes to go.mod
• divide memory_store RPC server to multiple files by handlers, as new one would be added and single file feels to be too big for that

[umputun]

LGTM