policy: Lower default relay fee to 0.0001/kB.

[davecgh]

This lowers the default minimum relay fee to 0.0001 DCR/Kb from its previous value of 0.001 DCR/Kb and increases the high fee multiplier to keep the same default high fee threshold of 1 DCR/kB.

In order to keep the comments accurate, it also updates the comments in the isDust function and the sample config file to match the new default relay fee as it's nice to have the most recent values as a reference in the comments.

Finally, it updates the tests for the expected new values as a result of the reduced default relay fee and adds add a couple of dust tests just below and above the dust point for the new reduced default relay fee.

It should be noted that this is only a default node policy change and as such does not affect the consensus rules in any way.

[JFixby]

@davecgh there could be an issue with ; minrelaytxfee=0.0001 parameter in sample config:

1. Someone has the old value in his config
2. He updates the code/dcrd version
3. forgets (because doesn't know about it) to update the parameter in his config and keeps the old value

suggestion: introduce a new parameter name for the minrelaytxfee=0.0001 and deprecate the old one