Is your feature request related to a problem? Please describe.
With the most recent changes to address thread safety related issues, the performance of SeqRepo while running with the default fd_cache_size=0 has been greatly impacted. At the moment the fd_cache_size parameter needs to get passed as an argument when creating a new SeqRepo instance. That makes it somewhat difficult to change for libraries that already depend on SeqRepo.

Describe the solution you'd like
To make it easier to modify the fd_cache_size behavior it would be great to be able to modify this via an env variable. Something like SEQREPO_LRU_CACHE_MAXSIZE.

Additional context
Currently the default behavior has a 10x slower performance compared to seqrepo v. 0.6.4. That's somewhat problematic and perhaps the default should not be 0 for the cache size?