MDEV-5825: Assertion `! is_set() || m_can_overwrite_status' fails in Diagnostics_area::set_error_status on executing rpl.rpl_parallel test

In parallel replication, there was an error case where we could call
my_error() in-between events. This causes the assertion, as the previous event
has reported ok status, but the following event has not yet reset the
diagnostics area. This happened when a worker thread detects that the SQL
driver thread is aborting, and when it gets an error from a prior commit at
the same time in wait_for_prior_commit().

Since this is already an error case, the code should be using
unregister_wait_for_prior_commit() instead of wait_for_prior_commit(). But
unregister is already done a bit later (from finish_event_group()), so just
removing the redundant call to wait_for_prior_commit() fixes the issue.

MDEV-5781 Item_sum_std::val_real(): Assertion `nr >= 0.0' fails on query with STDDEV_POP, ROUND and variable

- MDEV-5689 ExtractValue(xml, 'substring(/x,/y)') crashes
- MDEV-5709 ExtractValue() with XPath variable references returns wrong result.

Description:

1. The main problem was that that nodeset_func->fix_fields() was
called in Item_func_xml_extractvalue::val_str() and
Item_func_xml_update::val_str(), which led in some cases to
execution of the XPath engine *before* having a parsed XML value.
Moved to Item_xml_str_func::fix_fields().

2. Cleanup: added a new method Item_xml_str_func::fix_fields() and moved
most of the code from Item_xml_str_func::fix_length_and_dec()
to Item_xml_str_func::fix_fields(), to follow the usual Item layout.

3. Cleanup: a parsed XML value is useless without the raw XML value
it was built from.

Previously the parsed and the raw values where stored in separate String
instances. It was hard to follow how they are synchronized.
Added a helper class XML which contains both parsed and raw values.
Makes things easier to read and modify.

4. MDEV-5709: const_item() could incorrectly return a "true"
result when XPath expression contains users/SP variable references.
Now nodeset_func->const_item() is also taken into account to
catch such cases.

5. Minor code enhancements.

A huge number in the "day" part of an interval made the code to return
a negative date erroneously. Adding a test to return an error on a too
large "day" value.

After constant table row substitution the where condition may be converted
to always true. The function calculate_cond_selectivity_for_table() should
take into account this possibility.