This patch shares (for each log file) the `RotatingFileHandler` for
logging across all modules, reducing the number of open file descriptors
from ~78 to 1.
I had originally assumed this issue was a resource leak, but it seems to
just be a large fixed number of file descriptors. So this change mostly
just (1) shrinks the number of open file descriptors to a reasonable
level and (2) prevents two modules in the same process competing to
write to a log file.
