Long runs of the data-provider can cause collisions with filenames. Files were being
generated with 5-character names from a-z + 0-9, which should be fine, but it seems
the random number seeds aren't truly random, and you can get collisions with as few as
a few thousand files in a single block.

If you have fewer files per block you'd probably be OK, since the block names stand a
good chance of being better randomised. They have longer guids and are likely generated
further apart in time. So duplicate file-names across different blocks won't hurt.

The fix applied is to:
1) increase the filenames to 10 characters
2) append the unix epoch time in hex (8 characters) to the filename

 so names now look like .../dkeudhngof-01234567.root

 That ought to be good for at least several tens of thousands of files per second
Merge pull request #8 from TonyWildish/filename-uniqueness

Long runs of the data-provider can cause collisions with filenames.