Improve performance for custom business days (GH6584)
closes #6584

Tests are passing. Incrementing datetime with custom dateoffset is at about twice the speed now. This could probably be optimized more. 

can you add a vbench . look for agood place in the vb_suite dir
 Sure, might take a few days though. I'll let you know when I had the time to look into this. 
 I've added a small vbench to vbsuite/timeseries.py. 

Btw, what is the standard way to run vbsuite on the last commit? I did

%run test_perf.py -t 6305492393 -b 391ed321528 -r timeseries

timeseries_custom_bday_incr                  |   0.0157 |   0.0353 |   0.4437 |

Perhaps it would be good to add some hints on how to use vbench, which script to run etc. 
 https://github.com/pydata/pandas/wiki/Performance-Testing
 pls add a release notes entry under enhancements, referencing #6584
 I've added the release note and rebased. Do you want me to squash this into one commit? 

Also, I have started to implement `pandas.offsets.CustomBusinessMonth`. Should I open a separate PR for this or keep going here? 
 pls squash 

open a new PR/issue for different `CustomBusinessMonth`
 Ok done
 thanks!
