support explicit filtering of txs when preparing to mine

[antiochp]

This PR adds functionality to explicitly filter txs (by bucket) when preparing a set of candidate transaction to mine a block.

We now explicitly filter by kernel variant, allowing buckets containing transactions with the following feature variants (and only these feature variants) -

• Plain
• HeightLocked

HF3 will introduce the following new kernel variant (unsupported prior to HF3) -

• NoRecentDuplicate

When we introduce the proposed NoRecentDuplicate kernel variant in HF3 we will use this new filtering logic to explicitly start allowing txs containing NoRecentDuplicate kernels to be included in a candidate block to be mined.

TODO -

Later buckets can be dependent on earlier buckets due to fee calcs.

• So we need to hook into the "reject" logic earlier to filter out invalid txs and their dependent txs.

Actually tx_filter closure can be used on the way in to the txpool also. The filtering logic is the same.

• Use tx_filter on way in to txpool/stempool also.

[antiochp]

Jumping through a lot of hoops here to resolve a theoretical edge case where the txpool is in invalid state (contains NRD kernel prior to HF3 due to rewind).

When "preparing a block to mine" we verify the block will indeed be valid and in the unlikely case it is invalid we already fallback to mining an empty block.
So all this hoop jumping is not actually necessary.

Closing.