FSStructureServer: alternative access functionality

Access OMF object by pos, length and chunk data - expert use only.
Background for this development is a request by Andrew. He likes the idea
of keeping AFDB outside Mongo in big data blobs. However, for each uniprot
AC it's OK to store a couple of numbers => the numbers that are stored in
the indexing mechanism of FSStructure server (pos, length, chunk). This can
be quickly changed in case of an update without massive data shuffling.
Accessing an OMF object from a cold data blob then requires to open the
memory mapped data and ask the OS to jump to a certain location in that
data blob. This avoids loading the whole indexing data if only a single
structure needs to be loaded.