w-level general upgrades and as a complement to module-specific upgrade.
* Called on startup after initial XML parsing, but before module-specific upgrades
* Enabled by definign the `.ca_datastore_upgrade`
* [General-purpose upgrade documentation](https://clixon-docs.readthedocs.io/en/latest/backend.html#ge
neral-purpose)
* JSON parse error messages change from ` on line x: syntax error,..` to `json_parse: line x: syntax err
or`
* Unknown-element error message is more descriptive, eg from `namespace is: urn:example:clixon` to: `Fai
led to find YANG spec of XML node: x with parent: xp in namespace urn:example:clixon`.
* C-API parse and validation API more capable
* `xml_spec_populate` family of functions extended with three-value return values
* -1: error, 0: parse OK, 1: parse and YANG binding OK.
* `xml_parse` and `json_parse` API changes
* Three value returns: -1: error, 0: parse OK, 1: parse and YANG binding OK.
* Extended `xml_parse_file2` and `xml_parse_string2` extended API functions with all options available.
* New concept called `yang_bind` that defines how XML symbols are bound to YANG after parsing
* Existing API same except `xml_parse_file` `endtag` argument moved to `xml_parse_file2`
* C-API: Added instrumentation: `xml_size` and `xml_stats_get`.
* Fixed: Enabling modstate (CLICON_XMLDB_MODSTATE), changing a revision on a yang, and restarting made the backend daemon exit at start (thanks Matt)
* Also: ensure to load `ietf-yang-library.yang ` if CLICON_XMLDB_MODSTATE is set
* Yang Netconf leaf/leaf-list insert support
* For "ordered-by user" leafs and leaf-lists, the insert and value/key attributes are supported according to RFC7950 Sections 7.7.9 and 7.8.6
* Fixed RESTCONF api-path leaf-list selection was not made properly
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.
* 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].
* New xmldb_get1() returning actual cache - not a copy. This has lead to some householding instead of just deleting the copy
* xml_diff rewritten to work linearly instead of O(2)
* New xml_insert function using tree search. The new code uses this in insertion xmldb_put and defaults. (Note previous xml_insert renamed to xml_wrap_all)
