Fix crash in importwallet and dumpwallet formatting

- DecodeDumpTime was passing a statically allocated facet object to
  std::locale. However, "The constructed locale object takes over
  responsibility for deleting this facet object." causing a free()
  crash on scope exit. Fixes #3670.

- EncodeDumpTime was using the wrong format character for dates
  (appears accidentally introduced in 51ed9ec9)