MXS-2300: Add session command pruning

This commit adds a new parameter that, when enabled, prunes the session
command history to a known length. This makes it possible to keep a
client-side pooled connection open indefinitely at the cost of making
reconnections theoretically unsafe. In practice the maximum history length
can be set to a value that encompasses a single session using the pooled
connection with no risk to session state integrity. The default history
length of 50 commands is quite likely to be adequate for the majority of
use-cases.

server/modules/routing/readwritesplit/rwsplitsession.hh

@@ -182,6 +182,7 @@ private:
     void process_sescmd_response(mxs::SRWBackend& backend, GWBUF** ppPacket);
     void compress_history(mxs::SSessionCommand& sescmd);
 
+    void prune_to_position(uint64_t pos);
     bool route_session_write(GWBUF* querybuf, uint8_t command, uint32_t type);
     void continue_large_session_write(GWBUF* querybuf, uint32_t type);
     bool route_single_stmt(GWBUF* querybuf);