Remove access to the global gArgs for the fastprune argument and
replace it by adding a field to the existing BlockManager Options
struct.
When running `clang-tidy-diff` on this commit, there is a diagnostic
error: `unknown type name 'uint64_t' [clang-diagnostic-error] uint64_t
prune_target{0};`, which is fixed by including cstdint.
This should eventually allow users of the BlockManager to not rely on
the global gArgs and instead pass in their own options.