- add sht_is_null route function - add sht_inc route function - add sht_dec route function

#### Pre-Submission Checklist - [x] Commit message has the format required by CONTRIBUTING guide - [x] Commits are split per component (core, individual modules, libs, utils, ...) - [x] Each component has a single commit (if not, squash them into one commit) - [x] No commits to README files for modules (changes must be done to docbook files in `doc/` subfolder, the README file is autogenerated)

#### Type Of Change - [ ] Small bug fix (non-breaking change which fixes an issue) - [x] New feature (non-breaking change which adds new functionality) - [ ] Breaking change (fix or feature that would change existing functionality)

#### Checklist: - [ ] PR should be backported to stable branches - [x] Tested changes locally - [ ] Related to issue #XXXX (replace XXXX with an open issue number)

#### Description Implementing the identical functions from KEMI in the route functions.

You can view, comment on, or merge this pull request online at:

https://github.com/kamailio/kamailio/pull/4499

-- Commit Summary --

* htable: feature parity with kemi funcs

-- File Changes --

M src/modules/htable/doc/htable.xml (2) M src/modules/htable/doc/htable_admin.xml (105) M src/modules/htable/htable.c (95)

-- Patch Links --

https://github.com/kamailio/kamailio/pull/4499.patch https://github.com/kamailio/kamailio/pull/4499.diff