snapshot memory to disk, new from snapshot - disk to memory

[backurs]

Current repo only allows to snapshot bftrees that are disk-based. If we want to snapshot a bftree that is memory-based, we get a segmentation fault.

This PR implements functionality to snapshot in-memory bftree to disk. It simply reads all records from the in-memory bftree, creates an on-disk empty bftree, where it writes all the records.

It also creates a complementary function that takes as an input a path to a snapshot-ed bftree, reads it to create a disk-based bftree, reads all entries from it, creates an empty in-memory bftree and writes all the entries to the in-memory bftree.

While working on this PR I encountered a compilation error in file src/tests/concurrent.rs - the compiler complained about uninitialized vector in these lines:

            let mut buffer = Vec::with_capacity(4096);
            unsafe { buffer.set_len(4096) };

So I replaced these lines with

            let mut buffer = vec![0u8; 4096];

which I think is equivalent.