Disable flow control logic for sessions other than PROXYSQL_SESSION_MYSQL

[renecannao]

Flow control logic (see #1034) makes sense only on sessions with type=PROXYSQL_SESSION_MYSQL .

Retrieving a large resultset from Admin at maximum speed can cause crash.

[renecannao]

Example in 1.4.13:

(gdb) bt
#0  0x000000000056de50 in MySQL_Data_Stream::remove_pollout (this=0x7fffef220200) at mysql_data_stream.cpp:369
#1  0x000000000047bdcd in MySQL_Session::writeout (this=this@entry=0x7fffef21b000) at MySQL_Session.cpp:507
#2  0x0000000000488f58 in MySQL_Session::handler (this=this@entry=0x7fffef21b000) at MySQL_Session.cpp:3162
#3  0x00000000004a03f9 in child_mysql (arg=<optimized out>) at ProxySQL_Admin.cpp:3083
#4  0x00007ffff7bc16ba in start_thread (arg=0x7fffefdff700) at pthread_create.c:333
#5  0x00007ffff653941d in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:109

[renecannao]

Crash reproducible running this:

mysql -q -u admin -padmin -h 127.0.0.1 -P6032 -e 'SELECT replace(hex(zeroblob(50)), "00", a.Variable_Name) FROM stats_mysql_global a JOIN stats_mysql_global b JOIN stats_mysql_global c JOIN stats_mysql_global d LIMIT 500000;' > /dev/null