partial filename too long fix and resumable rebalances

[r0b0tb4c0n]

as a low-hanging fruit solution for the filename too long error, the filename suffix was changed from .balance to .ZIR -- (Z)fs (I)nplace (R)ebalance. a possible long-term solution would be to use something like LC_ALL=C tr -cd 'A-Za-z0-9' < /dev/urandom | head -c 32 to generate a random name as a uuid

enable resumable rebalances by adding a --resume flag that does the following:

1. gets the last file listed in rebalance_db.txt
2. filters everything up to and including the last file out of grouped_inodes.txt and writes the difference to resume_grouped_inodes.txt
3. sets the variable inode_file to be either grouped_inodes.txt if not resuming or resume_grouped_inodes.txt if resuming for later use in the script

[Baenwort]

I like the idea of using a shorter .ZIR instead of .balance!