Guidelines for tag lifecycle management

1) Properties are expected to remain consistent. This means that:

- no user code should call a setXXX() accessor of a tag handler
- the tag handler's code itself should not modify properties that are
set by a setXXX() method
- tag handlers should not perform invocation-specific logic in a
setXXX() method. That is, the setting of a property should
have no side effects.

2) Private, invocation-specific state must be managed manually.
Implications include that:

- release() is not necessarily called between invocations, which
means that tag logic should not count on private invocation-
specific state being reset by release()
- doEndTag() is not necessarily called at the end of every invocation
(in cases of abnormal termination -- e.g., an exception thrown inside
a tag's body or by one of its methods)
- private invocation-specific state is thus best initialized in
doStartTag()

- doFinally() *is* always called for tag handlers that implement
TryCatchFinally, so this method should be used if any invocation-
specific resources need to be released
- release() *is* always called at least once for a tag handler before
it is garbage-collected, so this method can and should be used to
release any long-term resources