Subject coinbase spends to -limitfreerelay
Subject coinbase spends to -limitfreerelay

Previously we were forwarding all coinbase spends however this may
not be desireable by some users that want/need to completely prevent all
traffic below a certain fee rate by setting their -limitfreerelay=0.

There is also a potential attack vector here which is prevented in that
some attacker could get a hold of coinbase coins and use them to create
large amounts of spam transactions which would then get around any
rate limiting attempts by node operators.

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