Without setting JEMALLOC_OVERRIDE, we end up linking to two different
jemalloc builds. Once dynamically, as a transitive dependency through
rocksdb, and a second time to the static jemalloc that tikv-jemalloc-sys
builds.
Some of the improvements here include:
* rocksdb can actually use jemalloc now instead of just pulling in a
second rocksdb for no reason
* "complement-runtime" factored back out into shell file
* complement image no longer uses `mkDerivation` for `copyToRoot`
because that's what `buildEnv` is for
* complement image no longer sets `SERVER_NAME`, complement already does
that
* all packages were factored out into `callPackage`-able files for use
with a custom `lib.makeScope pkgs.newScope`
* new version of `mkPackage` has options that are easier to use and
override such as `features`
