Pre-allocate NoTransactionException in TransactionContextManager

[mp911de]

NoTransactionException should be pre-allocated in org.springframework.transaction.reactive.TransactionContextManager to prevent expensive exception creation when calling TransactionContextManager.currentContext(). Ideally, this exception captures no stack trace, that is irrelevant in the context of reactive flows.

/cc @simonbasle @bsideup

[bsideup]

Consider using a stackless exception instead ( http://shipilev.net/blog/2014/exceptional-performance/#_conclusion )

Pre-allocated (aka cached, aka static) are hard to deal with in the debug mode (where we mutate them, necessary evil)