MINOR: cli/pools: add sorting capabilities to "show pools"

MINOR: cli/pools: add sorting capabilities to "show pools"

The "show pools" command is used a lot for debugging but didn't get much
love over the years. This patch brings new capabilities:
  - sorting the output by pool names to ese their finding ("byname").
  - sorting the output by reverse item size to spot the biggest ones("bysize")
  - sorting the output by reverse number of allocated bytes ("byusage")

The last one (byusage) also omits displaying the ones with zero allocation.

In addition, an optional max number of output entries may be passed so as
to dump only the N most relevant ones.