Empties the entire specified hash table

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#### Description <!-- Describe your changes in detail --> Add a new RPC comand to flush (empty) a htable. You can view, comment on, or merge this pull request online at:

https://github.com/kamailio/kamailio/pull/1828

-- Commit Summary --

* htable: Add RPC htable.flush <htable>

-- File Changes --

M src/modules/htable/doc/htable_admin.xml (23) M src/modules/htable/htable.c (24)

-- Patch Links --

https://github.com/kamailio/kamailio/pull/1828.patch https://github.com/kamailio/kamailio/pull/1828.diff