Allow string argument in AbstractUpdateAction::setRefresh

[knsarsaba]

Ref #1475

Currently, AbstractUpdateAction::setRefresh only accepts boolean values:

• true - to perform an immediate refresh
• false - to not refresh at all

The documentation, however, strongly suggests the usage of wait_for option to ensure a good cluster performance while doing the refresh operation. But the problem is that wait_for is of type string; thus, we cannot pass it as an argument to the method.

This modifies AbstractUpdateAction::setRefresh so that arguments of type string such as wait_for can also be passed to the method. Additionally, the return type of its getter counterpart, AbstractUpdateAction::getRefresh, has also been modified to mirror the accepted types of the setter.

[ruflin]

@knsarsaba Thanks for this contribution. It is so nice to be able to just merge such perfect contributions!