cfg: Added initial persist_state generating part.

Drivers can register a callback function for creating initial state for the
driver in persist_state. This function is called during init time, before
*_init is called.
logproto: Can set initial values for buffered server state.

It is needed in affile, to be able to skip old messages, we have to
set the reading position to the current file size.
affile: Implemented skipping old messages for affile.
logproto: Added presenter for LogProtoBufferedServerState
persistable-state-presenter: register constructors for persistable states.

There is a global init for persistable_state_presenters in which
the constructors can be registered.
persist-tool: Added persist-tool.

persist-tool can edit/dump/generate persist files with some limitations.