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:
- Simulator should also create this mbuf
- EPICS process uses hardcoded 64 MB for mbufs.
- 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.