testing/rocksdb: add static subpackage

Add a rocksdb-static subpackage to testing/rocksdb

The static library is build via a second invocation of make. Building the shared and static library with a single invocation of make turns on DEBUG mode and results in a build that is not suited for production use.

The static library does not get stripped by default. Since, for this package it makes a huge difference in size, the PR adds a call to strip for the static library.

It seems for other *-static packages apk info includes the static library in the contains field. I do not know why this does not seem to be the case with this package. I tested it with apk add rocksdb-static and seems to work fine.