Create an internal defaults package.

[jmalloc]

There are a lot of default values strewn throughout various sub-packages that interplay or are related in some way. They are also sort of duplicated in the infix package so that they appear in documentation and can be used to build the engine and network options.

It would be good to be able to see them in one place and define them in terms of each other.

We can do that in an internal package full of constants/variables, and reference this new package from both infix and the more specific packages.

This would be a good time to audit all of the default values too. For example, sql.DefaultMaxIdleConns is set to runtime.GOMAXPROCS(0) but should probably default to the same value as the concurrency limit which will likely be able to be considerably higher since the engine is certainly IO bound.