Adopt spdlog 0.16.0

[joaomoreno]

gabime/spdlog#570 (comment)

btw if you use aync logging, you might want to upgrade to 0.16.0 that will be released in the next few days.
Most notably in async logging, the back thread busy wait loop (when the queue is empty) will be reduced from 5/sec to 2/sec to reduce cpu waste.

Also, rotated log files will preserve their extension (“log.3.txt” instead of the current “log.txt.3”)

cc @sandy081

[sandy081]

@joaomoreno Adopted output panel to the change in rotating file format. Thanks.

[joaomoreno]

Verification: logs still work as expected