connectionmanager: avoid double lock

removeUnusedConnection can happen and will lock infosMtx_ then cbMtx_
and at the same moment onTlsNegotiationDone can happen and lock
cbMtx_ then infosMtx_ causing a potential double lock.
This patch avoid this case by avoiding the infosMtx_ to be locked
if destroying.

Change-Id: If21d35757e9dbd667ef3e20e8c13025479494333