As others here have mentioned, there is no necessary relationship between the awful async syntax so common to JavaScript environments and non-blocking IO. There are plenty of languages/environments that support highly scalable network operations (that is, do not use a new thread or process to handle each new connection) without introducing callback hell.

The Greenhouse framework in Python is one of them (http://teepark.github.io/greenhouse/master/). The docs (linked) have a concise discussion of the various approaches to parallelizing IO operations with pros and cons of each, and, for most database applications, an obvious winner.