Fix concurrent evaluation of static variables used from xquery:fork-join

[GuntherRademacher]

Static variables are evaluated at most once per query, involving a circular dependency check, but this was triggered incorrectly when variable values were accessed and evaluated concurrently by multiple threads started via xquery:fork-join.

The changes here

• replace the dontEnter runtime check with query-local static-variable evaluation coordination
• share a staticVarWaiting dependency map between parent and child query contexts
• detect circular static-variable dependencies across:
  • same or nested query contexts
  • sibling contexts waiting on variables evaluated by each other
• make static variable values volatile so cached results can be read before entering synchronization
• add QueryException.Data and InputInfo.copy() so errors from failed static-variable evaluation can be reconstructed for waiting threads without sharing one mutable QueryException instance

Regression tests are added for #2678, including:

• concurrent access to main-module and imported-module static variables
• lazy static variables evaluated only once
• side effects from lazy evaluation fired only once
• circular dependencies across sibling fork-join branches
• nested fork-join self-dependency
• copied error reporting for concurrent circular failures