container/mts: Find PSI and metadata all in one go

[Alan Noble]

Currently clients wishing to find a PSI and extract metadata employ a combination of calls to FindPid and ExtractMeta, and the latter in turn calls FindPid again (unnecessarily). A function that combines both of the above would be more efficient and would remove the need for clients to manipulate MTS slice indices.

[Alan Noble]

IndexPid function implemented in https://bitbucket.org/ausocean/av/commits/4f0a5948edbdc6b4678d4caa6f369e67254d2945