metadata: use allocated memory to write metadata to an mbuf

metadata: use allocated memory to load channel data and write metadata structure to a new mbuf "_metadata"

mbuf structure defined in src/shmem/metadata.h

Some things left to do:

1. Simulator should also create this mbuf
2. EPICS process uses hardcoded 64 MB for mbufs.
3. Does not pass CRC function to metadata simple constructor.

Size of channel name in metadata channel structure increased to 64 chars. This may cause some issues downstream.