R1:72a062d8027b

Truncate Conduit 'clientDescription' so we don't overflow the column

Summary: We store the `arc` commandline in this 255-character column, but it can be more than 255-characters long. If it's huge, truncate it.

Test Plan:
Executed:

arc list --conduit-uri=http://local.aphront.com:8080/ --conduit-version 6.