addapt dCache pool to accordingly to Filesystem's READ-ONLY mode

[mksahakyan]

#9270: Is it possible to start a pool w/o its metadata DB? (https://rt.dcache.org/Ticket/Display.html?id=9270) ticket closed

and ticket

https://rt.dcache.org/Ticket/Display.html?id=10540

[XMol]

Hi dCache.org,

does a read-only pool also address the redundancy feature request?

I'm envisioning a setup where there are a number of writable pools for data upload/import and the same number of additional pools that are read-only, but access the same pool data directories. E.g. read-only-pool facilitates access on writable_pool's inventory. There are probably a myriad of edge-cases I don't know, while only one seems really problematic to me: writable_pool should not delete any files that are opened through read-only-pool. One would need a POSIX unlink, where the source becomes inaccessible for future use, but current operations are unaffected.