Add dedicated storage backend for NVM

Up to now we have stored chunks into non-volatile memory by using the POSIX I/O interface and assuming that the target storage backend has been formatted with DAX (daxfs mode). This, however, does not bring all the performance out of the newest NVM devices such as Intel Optane DC SSDs, since we are restricted by some POSIX semantics regarding the namespace that we create to access chunks. Fortunately, more performance can be (theoretically) gained by accessing the NVM devices in devdax mode, which gives us total freedom to decide how to store and manage chunks.

More information: https://nvdimm.wiki.kernel.org/