Assign values to enum lttcomm_sessiond_command

Assign values to enum lttcomm_sessiond_command

These command's code are part of a communication protocol so numbering
them is important.

Note that the RELAYD command should be in a seperate enum but until we
wrote the specs of the protocol with the commands value, this is fine
for now.

Fixes #397

Signed-off-by: David Goulet <dgoulet@efficios.com>