Supporting multiple values for a single parameter in fdbmonitor conf

This was previously addressed in #461 / #468, but there is a new use case that may require a more general solution. When we have the same parameter multiple times in a monitor conf file, only the last one is used. This causes problems for the new feature in FDB 6.1 that allows multiple public / listen addresses. I've attached an example monitor conf that uses two addresses, one of which is using TLS and one of which is not using TLS.

[foundationdb.conf.txt](https://github.com/apple/foundationdb/files/3150156/foundationdb.conf.txt)

Start by reviewing issues #461 and #468 and the attached foundationdb.conf.txt example, then trace how fdbmonitor reads repeated parameters. Confirm how the two public/listen addresses are represented, including differing TLS settings. Done means repeated parameters are preserved and both addresses are accepted without regressing existing monitor configuration behavior.

Written by the indexing model from the issue text.