An alternative implementation (replacing the one based on repertoire).

This implementation makes Field send itself to Protocol_text using
data type specific Protocol methods rather than field->val_str()
followed by protocol_text->store_str().

As now Field sends itself in the same way to all protocol types
(e.g. Protocol_binary, Protocol_text, Protocol_local),
the method Field::send_binary() was renamed just to Field::send().

Note, this change introduces symmetry between Field and Item,
because Items also send themself using a single method Item::send(),
which is used for *all* protocol types.

Performance improvement is achieved by the fact that Protocol_text
implements these data type specific methods using store_numeric_string_aux()
rather than store_string_aux(). The conversion now happens only when
character_set_results is not ASCII compatible character sets
(e.g. UCS2, UTF16, UTF32).

In the old code (before any MDEV-23162 work, e.g. as of 10.5.4),
Protocol_text::store(Field*) used val_str() for all data types.
So the execution went through the character set conversion routines
even for numeric and temporal data types.

Benchmarking summary (see  details in MDEV-23478):

The new approach stably demonstrates additional improvement comparing
to the previous implementation (the smaller time - the better):

Original   - the commit before MDEV-23162
             be98036f

        1m9.336s
        1m9.290s
        1m9.300s

MDEV-23162 - the repertoire optimization

        1m6.101s
        1m5.988s
        1m6.264s

MDEV-23478 - this commit

        1m2.150s
        1m2.079s
        1m2.099s