Pipeline queries when possible

[bgrainger]

When resetting a connection, a "Reset Connection" packet is sent, then SET NAMES utf8mb4; to fix the collation (which is not reset properly). For most MySQL Servers, those two packets could be pipelined, which would halve the latency of resetting a connection.

This was previously done as #483 but reverted for #486 because it's not compatible with Aurora.

Consider adding a new connection string option that enables pipelining.

• Should it be enabled by default, with opt-out for Aurora?
  • Or should it be optional, for users who want higher performance?
• Can we detect Aurora and automatically opt out?

[bgrainger]

I can reproduce the hang with "5.7.mysql_aurora.2.10.1", which identifies itself as 5.7.12, but not with "8.0.mysql_aurora.3.01.0", which identifies itself as 8.0.23.

So a possible heuristic would be checking for ServerVersion.StartsWith("5.7.") && Server.EndsWith(".rds.amazonaws.com"). It's not foolproof, as it would also catch non-Aurora MySQL RDS instances, but the downside is simply slower connections.

If we can enable this heuristic only when Pipeline Queries isn't specified in the connection string, RDS users could override the heuristic by explicitly adding Pipeline Queries = True;.

[bgrainger]

Since pipelining is a great optimisation that should be on by default, the proposed connection string option is Disable Pipelining (aka DisablePipelining).

Similar to Pooling, the connection string option will be Pipelining with a default of True.