Access to empty vector by using operator[] may cause stdlib++
assertions to fail. Replaced the vector data access to use data()
method which is valid operation even if the vector is empty.
Added unit test to reproduce assertion with empty mutable_buffer access.
Added -D_GLIBCXX_ASSERTIONS preprocessor option to debug builds
to catch standard library misuse.
Added gcc 8 and gcc9 into travis build matrix.
- populate and pass real error description buffer to provider in case
of applying error
- return 0 from server_state::on_apply() if error voting confirmed
consistency
- remove fragments and rollback after fragment applying failure
- always release streaming applier on commit or rollback
