StoreArgument class getter method opens repo in read/write and unsafe MMAP mode

It seems that the getter method hasExternalBlobReferences() for a few implementations in scope of the following commit introduced a full source repo access routine that can cause serious lock issues (especially when opened in MMAP mode for Windows environments).

Moreover, reinitialisation of StoreArguments leads to OverlappingFileLockException.

Such getter method should only open repository in read only mode (using buildReadOnly() of FileStoreBuilder) method.
Considering setting MMAP always to false might be also a good idea to avoid buffer leakages (as getter should require as few features as possible) to be compatible.