Boost API: hook_boost_is_cacheable: have a way to know if the current page is cacheable for the status block

In order to get information about whether the current page is in the cache, we need to add a parameter to boost_boost_is_cacheable($parts) so that we can get more information on the reason why it's called.. more precisely, to know if it's the status block which is asking, because in that case, we don't want to return false because the user is logged in.

Related to : #1448476: Admin status block in 7.x-1.x

This is an API change:
function boost_boost_is_cacheable($parts)

Becomes:
function boost_boost_is_cacheable($parts, $request_type = 'normal')

How "request_type" is used might change in the future, since this could potentially solve other problems too.