Add maxTransactionsInFlight to IDMapEntry and OMIdMapEntry

[hcook]

This PR adds a maxTransactionsInFlight field to IDMapEntry and OMIdMapEntry. This field offers an upper-bound on the number of transactions possibly in flight from a given master associated with a range of ids. The calculation is protocol specific; while it is exactly the number of source ids for TileLink, for AXI4 incomplete information forces us to make a conservative overestimate: assume that every single ID could possibly generate the maximum number of transactions (which may or may not be true), and that each source could generate simultaneous read and write transactions using the same ID (which may or may not be true).

Related issue:

Type of change: feature request

Impact: API addition (no impact on existing code)

Development Phase: implementation

Release Notes

[richardxia]

LGTM, with a minor nitpick about backwards compatibility and public/privateness, which are more for my edification about Scala best practices than anything else.