Changes to use mocks in tests

[sivachandran]

The changes replace the use of mocky.io or real dependencies(e.g., RabbitMQ, Postgres servers) in tests with their mock versions.

Summary of changes

• Used mock for RabbitMQ, Kafka, Postgres and HTTP servers
• Moved DB mock from services to repository subpackage
• Introduced memory-based DB MemoryDB to use in handlers subpackage tests
• Fixed a bug in HTTPVal population code which was skipping the last header
• Corrected some of the tests where the assertions were not correct, e.g., testing len(UUID) which is always true.

Addresses #100