consistent option management

[emaxerrno]

we have 2 types of patterns

struct foo_opts {}
struct foo {
    foo (foo_opts) {}
}

and

struct foo {
    struct opts {}
    foo(foo::opts) {}
}

for lots of filesystem components.

consistently move them inside or outside.

Same for

struct foo {
    struct opts {}
    struct metrics {}

    private:
    metrics stats_;
}