When retrieving large result sets containing DECIMAL columns from a
secondary engine, conversion of DECIMAL values to strings for
transport over the text protocol is a bottleneck.

This patch improves the performance of the DECIMAL to string
conversion as follows:

- decimal2string() uses the optimized write_digits() and
count_digits() functions introduced in bug#30528427.

- Protocol_text::store_decimal() no longer uses an intermediate local
buffer, and instead writes the string directly into the destination
buffer.

- Protocol_text::store_decimal() uses decimal2string() directly
instead of going through the my_decimal2string() wrapper. The extra
code in the wrapper is only useful if the destination buffer could be
too small to hold the full string, and store_decimal() has allocated a
large enough buffer to hold the widest DECIMAL value.

Microbenchmarks (64-bit, Intel Core i7-4770 3.4 GHz, GCC 8.3.0):

BM_Decimal2String                    3908 ns/iter -> 2374 ns/iter
BM_Decimal2Bin_10_2                    19 ns/iter ->   17 ns/iter
BM_Protocol_binary_store_decimal      119 ns/iter ->   52 ns/iter

Change-Id: I76867b3e7ae93251c2fae716c39fc2f204d8a9c8