Retention to be based on number of messages

**Is your enhancement request related to a problem? Please describe.**
Currently I can only specify the size and time for retention.

**Describe the solution you'd like**
I would like pulsar to provide a mechanism for me to say "keep n messages"

**Describe alternatives you've considered**
Utilizing an amount of storage space as a retention policy roughly equal to n X message_size

The issue names no files, tests, or entry points. Start by locating Pulsar's existing size- and time-based retention configuration and implementation, then review related tests. Done means a retention policy can specify a message count and its interaction with the existing policies is covered.

Written by the indexing model from the issue text.