Fix: Incorrect Fee Estimation Due to Omitted estimate_mode in Bitcoin Core 28.0

[juanmigdr]

Fixes #1262

[farukterzioglu]

LGTM It introduces a behaviour change.

The previous behaviour was:

If the user doesn't set estimateMode
use Bitcoin Core's default value.
It means;
if node < 0.28, feeMode is conservative
else if node >= 0.28, feeMode is economical

we are changing the behaviour to;

If the user doesn't set estimateMode
feeMode is set to conservative by code.
Bitcoin Core's default is ignored.

Users that don't set estimateMode and have been using node >= 0.28 will be affected by a behaviour change if we merge this as is. Their fee estimation mode will be switched from economical to conservative , while Bitcoin Core did exactly the opposite.

With the current NBitcoin version, users with a node >= 0.28 already switched to economical as expected by Bitcoin Core.
This change will revert that silently.

I suggest to change the parameter estimateMode as nullable, remove the default value, and add to the parameters if it is not null.

[juanmigdr]

Thank you @farukterzioglu for your comment, I can see this will impact users already using a node with a version >= 0.28. I will make adjustments to my PR to cater for this.

[juanmigdr]

I have now made changes through this commit which should cater for the following scenarios:

If the user does not set estimateMode:

• If node < 0.28, NO estimateMode is sent in RPC -> We use default "conservative" mode - SAME
• if node >= 0.28, 'NO estimateMode is sent in RPC -> We use default "economical" mode - SAME

If the user sets estimateMode to conservative:

• If node < 0.28, NO estimateMode is sent in RPC -> We use default "conservative" mode - SAME OUTCOME - we just pass the estimateMode=conservative in the request
• if node >= 0.28, 'NO estimateMode is sent in RPC -> We use default "economical" mode - DIFFERENT OUTCOME - we pass the estimateMode=conservative in the request which is then used as the estimationMode

If the user sets estimateMode to economical:

• If node < 0.28, economical estimateMode is sent in RPC -> We use "economical" mode - SAME
• if node >= 0.28, economical estimateMode is sent in RPC -> We use "economical" mode - SAME