Add query timeouts using context cancellation

[amotl]

About

In order to improve the situation with GH-33, this patch enables timeouts for read and write operations, and adds the capability to configure the timeout values, defaulting to five seconds. The corresponding configuration settings are read_timeout and write_timeout.

Background

In production applications, it is always preferred to have timeouts for all queries: A sudden increase in throughput or a network issue can lead to queries slowing down by orders of magnitude.

Slow queries block the connections that they are running on, preventing other queries from running on them. We should always set a timeout after which to cancel a running query, to unblock connections in these cases.

-- https://www.sohamkamani.com/golang/sql-database/#query-timeouts---using-context-cancellation