[multifile]paxos_ledger_hook(+Action, 
?Key, ?Gen, ?Value, ?Holders)Hook called for all operations on the ledger. Defined actions are:
- current
 - Enumerate our ledger content.
 
- get
 - Get a single value from our ledger.
 
- create
 - Create a new key in our ledger.
 
- accept
 - Accept a new newly proposed value for a key. Failure causes the library 
to send a NACK message.
 
- set
 - Final acceptance of Ken@Gen, providing the holders that 
accepted the new value.
 
- learn
 - Accept new keys in a new node or node that has been offline for some 
time.