introduce multicore load initial corpus

[R9295]

implements and closes #1746 I ended up implementing a multicore version unlike what I wrote in the issue. It traverses the corpus entries twice (at most), which is inefficient but is probably the simplest implementation I could think of.

to test:

cd fuzzers/libfuzzer_libpng_norestart
RUST_LOG="debug" LIBAFL_DEBUG_OUTPUT=1 ./fuzzer_libpng --cores 1,2 --input seeds > debug
cat debug