Problem: Defrag was implemented before custom bolt options were added.
Currently defrag doesn't restore backend options.
For example BackendFreelistType will be unset after defrag.
Solution: save bolt db options and use them in defrag.
The io/ioutil package has been deprecated as of Go 1.16, see
https://golang.org/doc/go1.16#ioutil. This commit replaces the existing
io/ioutil functions with their new definitions in io and os packages.
Signed-off-by: Eng Zer Jun <engzerjun@gmail.com>