Rounding errors in estimatefee/estimatesmartfee return fees lower than the relay fee

For example calling `estimatefee 24` I get:

`0.00000999`, that is lower than `DEFAULT_MIN_RELAY_TX_FEE`.

Here is the commit that fixes this issue https://github.com/bitcoin/bitcoin/commit/e542728

Start with the estimatefee and estimatesmartfee entry points and compare their fee rounding behavior with the referenced Bitcoin commit e542728. Confirm that the returned fee is not below DEFAULT_MIN_RELAY_TX_FEE, using the reported estimatefee 24 case as the regression scenario.

Written by the indexing model from the issue text.