clear path cache after getting a proper config file (fixes #2605)

Signed-off-by: Jonas Schnelli <jonas.schnelli@include7.ch>
Merge pull request #2608 from jonasschnelli/pathCacheFix

clear path cache after getting a proper config file (fixes #2605)
Remove flaky util thread unit tests
Define dust transaction outputs, and make them non-standard
CreateTransaction: return strFailReason on failure
Un-hardcode TX_FEE constants

Allow setting of MIN_TX_FEE / MIN_RELAY_TX_FEE with
-mintxfee / -mintxrelayfee

Default values are the same (0.0001 BTC).