Improve modules logging

[RiccardoM]

Context

Currently the SDK modules are logging useful information using the Logger interface. This allows the node to print different levels of information when running (eg. debug, info, etc).

To make it easier for node operators to understand how their node is doing, we should implement logging into our modules as well when needed. This can be done by defining a new method for each keeper as follows:

func Logger() log.Logger {
  return ctx.Logger().With("module", fmt.Sprintf("x/%s", types.ModuleName))
}

Then, we can simply call the various methods (Info, Debug, ...):

logger.Info("asserted all invariants", "duration", diff, "height", ctx.BlockHeight())

[leobragaz]

@RiccardoM Should we also add logs to existent methods?

[RiccardoM]

@bragaz Yes, logs should be added with different levels (Info, Warn, Debug etc) when we think it would be nice to have them