Handle conn errors gracefully

For telnet pang, enter reconnect state
immediately, without crashing the process
and leave the pool.

For dummy metric pang, indicate an error
but leave the telnet connection intact.

Do not crash on gen_tcp:send, return the
error instead.
Be more explicit about closing the socket
Measure telnet ping/pong
Measure writes OK/NOK
Return proper reply to the caller
Make write failure schedule reconnect
Remove unused variable
Remove useless debug messages
Ignore ping calls when connecting
Re-pin dlhttpc
Wrap more details in kai_folsom
Change dlhhtpc opt to conform to latest version
Merge pull request #11 from aerosol/handle-tcp-errors-gracefully

Handle conn errors gracefully