Speedup compilation speed

libtorrent has a relatively heavy headers, that take lots of time to
process. This commit removes unnecessary includes of libtorrent headers
and replaces them with forward declarations. Also it optimizes
cross-header dependencies.

"time make" of full rebuild before this commit:

real    13m35.937s
user    12m1.295s
sys     1m25.908s

after:

real    10m54.390s
user    9m31.167s
sys     1m12.580s