[Closed in favour of !2837; V108-specific] Increment provider (not just node) bond with message RUNE when unbonding/leaving

[Closed for the time being in favour of !2837; please tell me if re-opening is preferred.]

[V108-specific]

Intended to close #1486
'RUNE in an Unbond or Leave transaction is treated like rewards (split between all providers), rather than incrementing the signing provider's bond'
and !2837 (to which this is an alternative)
'Fail validation of Unbond/Leave messages with a non-zero Coin amount'.