Another command already in progress error under heavy load

[eaydingol]

During periods of heavy query load and high IOPS, the following error occurs: "another command is already in progress." This error is preceded by multiple query timeouts, prompting Citus to send cancel requests to the workers. The PR introducing connection reuse also highlights this issue.

https://github.com/citusdata/citus/commit/24e06af6d20eb992b1fdb7b2d96b13a12750595e