Consider limiting redelegation/ubd entries as spam prevention

[rigelrozanski]

follow up to the implementation of #1402

I think we should consider a basic spam measure to limit the number of Entries in either Redelegation or Unbonding-Delegation to a staking parameter - for starters we could set it at say 50 (aka you can only have up to 50 incomplete unbonding delegations between a delegator-address/validator-address pair - per unbonding period) - thoughts? @cwgoes ?

[cwgoes]

That sounds fine; I think we can be way more conservative, e.g. 5-10.

[cwgoes]

We should absolutely do this prelaunch, I think we'd open ourselves up to spam otherwise.