Delete old checkpoints in stateDB automatically

Introduce a constructor parameter for the StateDB called `keep`, which tells
how many checkpoints to keep.  When doing a new checkpoint, if the number of
existing checkpoints exeeds `keep`, the oldest ones will be deleted.

cli/node/cfg.buidler.toml

@@ -14,6 +14,7 @@ APIAddress = "localhost:12345"
 
 [StateDB]
 Path = "/tmp/iden3-test/hermez/statedb"
+Keep = 256
 
 [PostgreSQL]
 Port     = 5432