get rid of static buffer

this buffer was used for two types of outgoing packets:
  1. local msg
    - msg can be big, there is is no apriori boundary -> use dynamic
      memory instead
  2. transceiver msg
    - no need to copy msg in this buffer -> transceiver send is blocking
tune CCNL_RIOT_CHUNK_SIZE to fit in CC1100 packets
make max_cache_entries a parameter
fix: remove oldest dynamic entry in content store
enable timeout events