Add -blockmintxfee setting

[zeptin]

Currently transactions that fall below the relay fee rate minimum of 0.0001 STRAX per KB can still be mined into blocks. However, the block construction has a further (currently hardcoded) limit of 0.00001 STRAX per KB. Transactions that are below this fee rate will never be included in blocks even if they make it into a node's mempool. This change makes this lower limit configurable, similarly to the -blockmintxfee setting in bitcoind. The default value has been kept.

It would have been nice to remove the PowMining.DefaultBlockMinTxFee value entirely and base the default minimum fee rate on the node relay policies, but the value gets used in several tests and it was cleaner to keep it for now to tie all the 'magic numbers' together. It is also unclear whether increasing the block fee minimum tenfold to match the relay minimum would have any adverse impact.

The dumpprivkey RPC command commits also seem to be missing from the 1.5.0.0 branch, so they have been brought in here.

[quantumagi]

@zeptin , Looks good. Is the dumpprivkey a possible security risk and if not why not?

[zeptin]

@quantumagi I don't think it changes the security assumptions of the node. Similarly to sending transactions (or staking) via RPC it requires the wallet to be unlocked beforehand in order to work. If an external entity has access to the node via RPC while the wallet is unlocked they could already make transactions, this PR doesn't change that. Note that the functionality of the RPC is similar to the privatekey endpoint in the wallet controller, although that endpoint explicitly requires a password to be provided.

[quantumagi]

Would it detract from the usability of this endpoint if we require an explicit password to be passed?

[zeptin]

Not much point, the node has an RPC password anyway. Adding a further password to the RPC parameters breaks compatibility:

https://developer.bitcoin.org/reference/rpc/dumpprivkey.html

[quantumagi]

LGTM