This commit simply turns all mutable data structures into concurrent ones.

The Triplequote Hydra Scala compiler may compile a same source file from
different workers. Hence, the assertion need to be remove or relaxed. After
discussing with @jvican, the assertion is moved behind a strictMode flag as it
might still be useful for debugging incremental compiler's issues.

The IncOptions#ignoredScalacOptions field is available since 1.2.0 not 1.3.0.
Had to correct this as otherwise MiMa reports the following issues:

```
[error]  * method
of(Int,Double,Boolean,Boolean,Int,java.util.Optional,java.util.Optional,Boolean,java.util.Optional,Boolean,Boolean,Boolean,java.util.Map,Boolean,xsbti.compile.ExternalHooks,Array[java.lang.String])xsbti.compile.IncOptions
in class xsbti.compile.IncOptions does not have a correspondent in current
version
[error]    filter with:
ProblemFilters.exclude[DirectMissingMethodProblem]("xsbti.compile.IncOptions.of")
[error]  * method
create(Int,Double,Boolean,Boolean,Int,java.util.Optional,java.util.Optional,Boolean,java.util.Optional,Boolean,Boolean,Boolean,java.util.Map,Boolean,xsbti.compile.ExternalHooks,Array[java.lang.String])xsbti.compile.IncOptions
in class xsbti.compile.IncOptions does not have a correspondent in current
version
[error]    filter with:
ProblemFilters.exclude[DirectMissingMethodProblem]("xsbti.compile.IncOptions.create")
```