using ""%" HEX HEX" mechanism for escaping from RFC 2396

#### 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

I consider it non breaking because with the existing a field containing an ASCII vertical bar '|' would break the separator count and result in broken CDRs, etc. in this case changing the default behavior is probably recommended, someone could abuse this flaw.

Test: consider the following message, the '|' character in the call-id will break the accounting event. ``` INVITE sip:+11231234567@127.0.0.101 SIP/2.0 Via: SIP/2.0/UDP 1.1.69.51:5070;rport;branch=z9hG4bKPjpM.BvWCQzoS0BJ7TgnuLF9YtmHQTlDgd Max-Forwards: 70 Call-ID: fG8qa5Xc-iNDLhIAnuLZP|njL1sojRRD ```

With the encoding it is replaced with '%7C', no modifications should be required to systems processing the ACC events since different behavior will only take place when there is an invalid ACC event: ``` cat /var/log/acc/acc_129.log INVITE|upDPn89PCIq7SLTssEGoLoSbC-jDRnfj||fG8qa5Xc-iNDLhIAnuLZP%7CnjL1sojRRD.|403|Forbidden ```

Extra comments: I corrected the documentation on the command 'flatstore.rotate', since it was not working, but this one is 'flatstore.k_rotate'

If I understand correctly this module was never migrated from SER to Kamailio completely ... This change may be the only important one for operation, so I am not proposing any extra refactoring. You can view, comment on, or merge this pull request online at:

https://github.com/kamailio/kamailio/pull/1262

-- Commit Summary --

* db_flatstore: encode delimiter param

-- File Changes --

M src/modules/db_flatstore/db_flatstore.c (5) M src/modules/db_flatstore/doc/db_flatstore.xml (14) M src/modules/db_flatstore/doc/db_flatstore_params.xml (11) M src/modules/db_flatstore/km_flatstore.c (15) M src/modules/db_flatstore/km_flatstore.h (2)

-- Patch Links --

https://github.com/kamailio/kamailio/pull/1262.patch https://github.com/kamailio/kamailio/pull/1262.diff