scheduler: Protect MimeDatabase from parallel access (fixes #466)

scheduler: Protect MimeDatabase from parallel access (fixes #466)

If multiple create_local_bg_thread threads are spawned, they can fight
over MimeDatabase (specifically over MimeDatabase->srcs array), ending
up in SIGSEGV if one thread uses the array, but the other deletes it.

I chose to use RW locks for protecting the pointer - IMHO we benefit
from multiple threads being able to read in parallel.