Change SOVERSION at API breaks

Since the SOVERSION doesn't need to follow the library's version and
simply needs to be monotonically increasing whenever we release
something that breaks the ABI, we can set some number and allow multiple
versions of the library to be installed side-by-side.

We start here with the minor version as that's what we release for now,
and it allows to backport this change to earlier versions.
Bump version to 0.21.0

Bump library version to 0.21.0 and SONAME to 21
Merge branch 'cmn/soversion' into development