More reliably get futures returned by ChangeIndexer

Convert futures to be CheckedFuture<?, IOException>s so calling
checkedGet() does not result in too many extra exception types. Rename
the methods to {index,delete}Async so it is clear that these just
launch a future. For all usages, make sure checkedGet() is invoked on
the future, either immediately or after another block of non-Change-
mutating code.

Change-Id: I18b55b0b03b2be0a70995a0b08a4fc3193db96e3
17 files changed