Handle BP claimrewards when circulating supply exceeds max supply

[DenisCarriere]

Purpose

When eosio.token::supply::EOS has reached or exceeds maximum supply, the claimrewards action will no longer function since it requires to issue new tokens to allocate BP rewards.

BP producer pay & vote pay should continue to be allowed to claim if eosio has sufficient EOS balance to distribute funds.

Scope

• allow existing eosio balance to be used to fund bpay & vpay.
• do not perform eosio.token::issue if circulating supply will exceed maximum supply.
• rename "unallocated inflation" to "unallocated bucket"

Reference

• claimrewards
• currency_stats

[DenisCarriere]

Implemented via: AntelopeIO/reference-contracts#66