[INTERNAL] limited concurrent thread access of BinaryChannelConnection

Only 32 threads can now use the send method in parallel. This should be
more than enough. This change also fixed an (theoretical) issue where
the fragmentId could collide if one thread sends an extra ordinary
amount of data while another thread only sends small chunks.

Change-Id: I8632d85bd3c6a9e06174ea76185d7424e2da53a4
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1512
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>