Perform a minimum size validation when creating oplog

Currently, when creating the oplog or other capped collections MongoDB allows you to create an oplog <16MB without warning or error. Hence the collection can be smaller than a single large document. Given that the collection must be dropped to correct this, we should validate the size and enforce a sane minimum (perhaps >32MB - i.e. greater than the size of two max sized BSON docs, should be disallowed and anything under 1GB should print a warning).