Speed up DatetimeConverter by using Matplotlib's epoch2num when possible.
Added vbench performance benchmark for speed up of DatetimeConverter.
Fixes to DatetimeConverter speed up following PR comments and corresponding note in release notes..
Fix failed tests when importing Series from pandas.
Merge pull request #6650 from agijsberts/speed-up-dateconverter

PERF: Speed up DatetimeConverter by using Matplotlib's epoch2num when possible...