multi: No stake height checks in check tx inputs.

[davecgh]

This relies on #1456.

This removes a redundant check for validating that votes and revocations are not in a block before they are allowed from CheckTransactionInputs since it has nothing to do with validation of transaction inputs and it's an impossible condition to hit in the block validation path because the same check is done much earlier in the validation process.

It should also be noted that the checks this removes are also more permissive than they need to be because the first vote is not allowed until stake validation height, which is after stake enabled height. There is no effect on consensus because, as previously mentioned, the more restrictive checks earlier in the process prevent these checks from ever being exercised.

However, the mempool previously relied on this duplicate check to reject early votes and revocations, so this adds the checks, with the corrected heights, directly to the mempool early in the process where they more naturally belong. It is more efficient to reject the transactions in that scenario earlier before doing more work as well.

This is work towards #1145.

[davecgh]

Test results from --simnet which has a StakeValidationHeight of 144:

2018-09-14 03:04:36.785 [INF] MINR: Block submitted via CPU miner accepted (hash 0055277f74c13adf5038e5a2d8b094ed0d2ad9e71e4e5f826fa3034ef43369f3, height 143, amount 346.53465346 DCR)
2018-09-14 03:04:36.959 [DBG] TXMP: Accepted vote cf512c66fe82098e37ba5bb591995948fe74c0e8346f0a479417d1ae1d44bee0 for block hash 0055277f74c13adf5038e5a2d8b094ed0d2ad9e71e4e5f826fa3034ef43369f3 (height 143), voting true on the transaction tree
2018-09-14 03:04:36.959 [DBG] TXMP: Accepted transaction cf512c66fe82098e37ba5bb591995948fe74c0e8346f0a479417d1ae1d44bee0 (pool size: 1)
2018-09-14 03:04:36.960 [DBG] TXMP: Accepted vote 945d57ae75dad40fe833a9e82b2ddf4c1b09142635bbfd9ee458fbfddb610506 for block hash 0055277f74c13adf5038e5a2d8b094ed0d2ad9e71e4e5f826fa3034ef43369f3 (height 143), voting true on the transaction tree
2018-09-14 03:04:36.960 [DBG] TXMP: Accepted transaction 945d57ae75dad40fe833a9e82b2ddf4c1b09142635bbfd9ee458fbfddb610506 (pool size: 2)
2018-09-14 03:04:36.961 [DBG] TXMP: Accepted vote ad71c9b80499052ebb60a35368c5bb317e05b41a1c76aeb61f077711c59e7e78 for block hash 0055277f74c13adf5038e5a2d8b094ed0d2ad9e71e4e5f826fa3034ef43369f3 (height 143), voting true on the transaction tree
2018-09-14 03:04:36.961 [DBG] TXMP: Accepted transaction ad71c9b80499052ebb60a35368c5bb317e05b41a1c76aeb61f077711c59e7e78 (pool size: 3)
2018-09-14 03:04:36.961 [DBG] TXMP: Accepted vote 8939719c2c97785f2b95600cfae54bf8f1890d6d5bd6c74a49b0c4096c60252c for block hash 0055277f74c13adf5038e5a2d8b094ed0d2ad9e71e4e5f826fa3034ef43369f3 (height 143), voting true on the transaction tree
2018-09-14 03:04:36.961 [DBG] TXMP: Accepted transaction 8939719c2c97785f2b95600cfae54bf8f1890d6d5bd6c74a49b0c4096c60252c (pool size: 4)
2018-09-14 03:04:36.962 [DBG] TXMP: Accepted vote 82c25c3012e0c12ca12061e77addbcd2f5afd4d1ce047a450e6cdecb6c972ba5 for block hash 0055277f74c13adf5038e5a2d8b094ed0d2ad9e71e4e5f826fa3034ef43369f3 (height 143), voting true on the transaction tree
2018-09-14 03:04:36.962 [DBG] TXMP: Accepted transaction 82c25c3012e0c12ca12061e77addbcd2f5afd4d1ce047a450e6cdecb6c972ba5 (pool size: 5)