Reduce memory footprint causes by MySQL_Data_Stream queue buffers

[renecannao]

WHY

MySQL_Data_Stream uses 2 queues and relative buffers.
Historically, they were used for both frontends and backends, but after recent development they are not used anymore by backend
In case of very large number of connections , this can cause a huge amount of memory utilization.
The good things is that is memory allocated but never used (not even initialized), therefore the Linux overcommits the memory, and proxysql has a lot of virtual memory but very little resident memory.

WHAT

• Do not allocate the buffers id the data stream is not of type MYDS_FRONTEND

[renecannao]

From prelim tests it seems that not initializing the queue will break other code, therefore for now we will proceed with a not elegant fix:

• always initialize the queues/buffers
• if myds_type is not MYDS_FRONTEND , deallocate the buffers associated with the queues