Profile memory usage while processing very large resultsets

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When processing very large resultsets and with a high level of concurrency, virtual memory grows very high.

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From a first investigation it seems that data is sent to the clients only once for each poll() request.
The consequence of this is that ProxySQL sends data to the client slower than it receives it from the backend, therefore will buffer a lot of resultsets internally and using a lot of memory.
Fixing this issue will result in:

• less memory burst
• better performance

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Current improvements:

• MySQL_ResultSet::buffer_to_PSarrayOut() calls realloc() if the buffer preallocated is too big: this will reduce memory footprint if data is not sent to client fast enough
• MySQL_Session::writeout() calls send() in a loop, trying to send as much data as possible to the client. This will reduce memory footprint as it tries to reduce the amount of data buffered, and it should also improve performance at scale