* Fixed multi-namespace for augmented state which was not covered in 4.2.0.
* The multi-namespace augment state may rearrange the XML namespace attributes.
* Mandatory variables can no longer be deleted.
* This occurs in cases where there are more than one XML namespaces in a config tree, such as `augment`:ed trees.
* Affects all parts of the system, including datastore, backend, restconf and cli.
* Invalid api-path syntax (eg non-matching yang) error changed from 412 operation-failed to 400 Bad request invalid-value, or unknown-element.
* New clixon-lib@2019-08-13.yang revision
* Bugfix: If `ietf-netconf.yang` was imported from any yang module, client/backend communication stops working.
* Restconf RFC 8040 increased feature compliance
* Cache-Control: no-cache added in HTTP responses (RFC Section 5.5)
* Restconf monitoring capabilities (RFC Section 9.1)
* Added support for Yang extensions
* New plugin callback: ca_extension
* Main backend example includes example code on how to implement a Yang extension in a plugin.
* JSON changes
* Non-pretty-print output removed all extra spaces.
* Example: `{"nacm-example:x": 42}` --> {"nacm-example:x":42}`
* Empty JSON container changed from `null` to `{}`.
* Empty list and leafs remain as `null`
* Removed unnecessary configure dependencies
* libnsl, libcrypt, libm, if_vlan,...
* pseudo-plugin added, to enable callbacks also for main programs. Useful for extensions
* Yang Unique statements with multiple schema identifiers did not work on some platforms due to memory error.
The reason is to be conservative with the API. However, less used functions, such as xpath_vec_bool(), xpath_vec_ctx() and xpath_vec_flag() are changed with a new `nsc`parameter, which should be set to NULL in most cases.
* Renamed startup_db_reset -> xmldb_db_reset (its a general function)
* In startup_extraxml(), check if reset callbacks or extraxml file actually makes and changes to the tmp db.
* Added regex cache to type resolution
* Added compiled regexp parameter as part of internal yang type resolution functions
* All internal `ys_populate_*()` functions (except ys_populate()) have switched parameters: `clicon_handle, yang_stmt *)`
* Added check for libxml in configure';
* Added clixon_util_regexp utility function
* Yang state get improvements
* Integrated state and config into same tree on retrieval, not separate trees
* Added cli functions `cli_show_config_state()` and `cli_show_auto_state()` for showing combined config and state info.
* Added integrated state in the main example: `interface/oper-state`.
* Added performance tests for getting state, see [test/test_perf_state.sh].
* Validate-only transactions are terminated by an `end` or `abort` callback.
* If a commit user callback fails, a new `revert` callback will be made to plugins that have made a succesful commit.
