Prep for 2.3.0.rc4

Maybe we defer some of these to 2.3.1 & 3.0.0 (master).

- [x] bump version to 2.3.0.rc4
- [x] fix time_select params? (#901)
- [x] merge #901 into 2.3-stable
- [x] documentation or something for #968 
- [x] merge #968 into 2.3-stable
- [ ] update CHANGELOG in 2.3-stable and master

Deferred to future releases:

- [ ] merge conditional validations branch (#992)
- [ ] fix mutex issues? (#904)