File descripor ID with 2 provided ways to compare between them:
1. is_mirror_of():
Test if identical:
For example: when LHS writes one byte to a file at X offset, RHS file be able to read that exact byte at X offset)
2. is_coherent_with():
Test if both files point to the same file:
For example: if a file descriptor pointing to the complete file exists and is being truncated to 0 bytes from non-
-zero size state: this has to affect both RHS and LHS files.
eecadab387