Diplomacy is not thread safe

https://github.com/chipsalliance/rocket-chip/blob/86a2f2cca699f149bcc082ef2828654a0a4e3f4b/src/main/scala/diplomacy/LazyModule.scala#L218-L224
makes Diplomacy not thread safe.
Which makes not possible to invoke multiple LazyModule inside a JVM
I'm proposing moving those variables to `DiplomacyCtx` class, but this will introduce a API change to entire codebase.

**Type of issue**: bug report

**Impact**: API modification

**Development Phase**: request

**Other information**

**If the current behavior is a bug, please provide the steps to reproduce the problem:**
Invoke `LazyModule` twice, `index` and `scope` will lead to strange behavior.

**What is the current behavior?**

**What is the expected behavior?**

**Please tell us about your environment:**

**What is the use case for changing the behavior?**

Start at the linked LazyModule.scala lines 218-224 and inspect how index and scope behave when LazyModule is invoked twice in one JVM. Review DiplomacyCtx as the proposed location for those variables, then verify that multiple LazyModule instances no longer produce strange behavior while accounting for the stated API change.

Written by the indexing model from the issue text.