Make subsidy halving interval for simnet 210000.
Make subsidy halving interval for simnet 210000.

This value matches testnet and mainnet and makes more sense because it
allows way more coins to be generated which is useful during simulation
testing.

NOTE: this will invalidate existing simnet chains, but since they
are only intended to be short lived for the duration of a simulation test,
this is a non-issue.

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