CDI Transactional interceptors are not thread safe. (previousUserTransactionAvailability)

The problem is with availability of `UserTransaction`.

Availability of UserTransaction is stored in `ServerVMClientUserTransaction.isAvailables` which is `ThreadLocal<Boolean>`.

`TransactionalInterceptorRequired` when intercepting call, stores thread local value of `isAvailables` in `TransactionalInterceptorBase.previousUserTransactionAvailability` field of interceptor - which is of type boolean(not thread local).

Here's a story:
We have a bean - `beanA` - with single method annotated transactional - `beanA.transactional()`.
This method is being called from two threads, where:

• Thread 1 is participating in BMT, therefore it's `ServerVMClientUserTransaction.isAvailables` value is true.
• Thread 2 is participating in CMT, therefore it's `ServerVMClientUserTransaction.isAvailables` value is false.
  Both threads call `beanA.transactional()` at the same time.

What may happen:
1. Thread 1 - enters method `beanA.transactional()`.
Interceptor sets previousUserTransactionAvailability to true.
2. Thread 2 - enters method `beanA.transactional()`.
Interceptor sets previousUserTransactionAvailability to false.
3. Thread 1 - exits method `beanA.transactional()`
Interceptor restores thread local value of `isAvailables` to false (leaked value from thread 2!)