SI-7982 Changed contract of askLoadedType to unload units by default

The rationale for not keeping units loaded by default is that the more
units are loaded, the slower is background compilation. For instance, in
the Scala IDE for Eclipse (which uses the presentation compiler),
typechecking occurs every time the reconciler kicks-in (~500millis after
you stop typing), hence it is important that units are not kept loaded
unless strictly necessary (for some extra information about this, see
https://www.assembla.com/spaces/scala-ide/tickets/1001388)

While I agree that using a boolean argument (`keepLoaded`) for deciding
if a unit should be loaded isn't a great design, other methods in
`CompilerControl` also have a keepLoaded parameter, so at least we have
some consistency. For the future, I'm thinking we should be able to
remove the `keepLoaded` flag altogether, and change the implementation
of `askLoadedType` to preserve the same units loaded in the presentation
compiler before and after its execution. Basically, if you want a unit
to be kept loaded, you should call `askReload` first, and then
`askLoadedType`. However, to reduce impact, I think the changes carried
by this commit will help us estimate if the solution I just outlined is
viable (because `askLoadeType` won't be keeping units loaded by default,
which wasn't the case with the former implementation).

(While the patch was mostly contributed by @huitseeker, @dotta has edited the
commit message to preserve the comments in the PR
https://github.com/scala/scala/pull/3209)