When retrieving large result sets containing many integers from a
secondary engine, conversion of the integers to strings for sending
over the text protocol is a bottleneck.

This patch improves the performance of some functions that are used by
the text protocol for converting integers, and also the integer
components of temporal values, to strings.

Protocol_text::store_{tiny,short,long,longlong,date,time,datetime}():

- Write the string value directly into the protocol buffer, instead of
  writing it to a stack buffer first and copying it to the protocol
  buffer.

int10_to_str(), longlong10_to_str():

- Write the string value directly into the destination buffer, instead
  of writing it to a stack buffer first and copying it to the
  destination buffer.

- Write two digits at a time, to reduce the number of relatively
  expensive modulo and division by 10 operations.

int2str():

- Removed. It had only one caller, which was updated to use
  longlong2str() instead.

longlong2str():

- Remove an optimization for VAX, which had a negative impact on the
  performance on modern hardware.

- Change some callers which requested conversion with base 10 to use
  longlong10_to_str() instead. longlong10_to_str() is specialized for
  base 10, and is faster than the generic variant.

my_date_to_str(), my_time_to_str(), my_datetime_to_str():

- Use the helper functions written for int10_to_str() and
  longlong10_to_str() instead of the similar helper functions in
  my_time.cc, and remove the helper functions in my_time.cc.

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

BM_longlong10_to_str                  34 ns/iter ->  16 ns/iter
BM_longlong2str                      140 ns/iter -> 106 ns/iter
BM_my_time_to_str                     23 ns/iter ->   9 ns/iter
BM_my_date_to_str                      6 ns/iter ->   3 ns/iter
BM_my_datetime_to_str                 26 ns/iter ->  10 ns/iter
BM_Protocol_text_store_tiny           11 ns/iter ->   8 ns/iter
BM_Protocol_text_store_longlong       37 ns/iter ->  18 ns/iter
BM_Protocol_text_store_date           12 ns/iter ->   6 ns/iter
BM_Protocol_text_store_time           27 ns/iter ->  13 ns/iter
BM_Protocol_text_store_datetime       30 ns/iter ->  13 ns/iter

Change-Id: I4e758e4ab9a218e02ed28da8dd3bdabe675fbfd6