DATAJPA-628 - Use a shared SpelParser instead of recreating it.

We now use a shared SpelExpressionParser and propagate this through the String-based JpaRepositoryQuery implementations to avoid reinstantiations.

Cleaned up ParameterBinder inheritance hierarchy by merging (and removing) ExpressionAwareParameterBinder into SpelExpressionStringQueryParameterBinder.

Original pull request: #121.
DATAJPA-628 - Polishing.

Made central SpelExpressionParser static.

Original pull request: #121.