Speed up DatetimeConverter by using Matplotlib's epoch2num when possible.
Added vbench performance benchmark for speed up of DatetimeConverter.
BUG: disallow mixed dtype operations
Merge pull request #6652 from cpcloud/gh-5198-stack-blow-eval-mixed-type-operations

BUG: disallow mixed dtype operations in eval/query
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...
REGR: fixing Timestamp/Series subtraction, resolves #6648