Update object finalizers and add keepalives - from indy-shared-rs

[swcurran]

The changes from this PR in the Python wrapper of indy-shared-rs needs to be done in this repo — https://github.com/hyperledger/indy-shared-rs/pull/26/files. A new release should be triggered for the Python wrapper immediately after. This is to address a bug in the verifiers using anoncreds-rs with the Python wrapper — verification failing when multiple revocation registries are used.

[swcurran]

@amanji -- this is a Python task as discussed last Friday. The Python wrapper for this repo should be about the same (give or take :-) ) as indy-shared-rs, so this should be a straight forward task, even though there is little detail. Do check on a test -- I didn't check today to be sure, but I recall that being one.