Reorganize pom

[turu]

This pr improves the structure of pom.xml by:

• adjusting it to follow Maven conventions (build section at the bottom)
• segregates Maven dependencies into 4 categories (to aid future work):
  • Core dependencies
  • Non-public Cloudera Hive & Impala connector dependencies
  • Public Cloudera Hive & Impala connector dependencies
  • Testing dependencies
• extracts liquibase-core version to a parameter
• fixes a typo in the artifact name: imapala -> impala
• applies small fixes to .gitignore

[eselyavka]

:+1: Could you please squash commits to one?

[turu]

@eselyavka done