BUG: remove FixValue, fix linting errors, move and complete get_status_description

Summary

Client-side fix (or rather clean-up) of caosdb-server#169 (closed).

This MR removes the FixValue work-around (which is needed until caosdb-server!27 (merged)) and adds the missing grpc::StatusCode mappings to caosdb::StatusCode (unrelated to caosdb-server!27 (merged), but it was convenient for debugging to do this now).

Additionally, it fixes some linter warnings in ccaosdb.

Focus

• Implementation of get_status_description moved from "transaction.cpp" to "status_code_description.cpp".
• FixValue and FixValueImpl have been removed.
• Nothing needed to be done here for the fix of the unit problems (#10 (closed))

Test Environment

• Integration tests in caosdb-cppinttest!13 (merged) for both #10 (closed) and caosdb-server#169 (closed)
• No manual testing needed I'd say

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