fix: heap-use-after-free in eventLoop()

it points to an element of the servers_ map. When we call
servers_.erase(it), we free this element. Currently this is done
before calling connectedPeers_.erase(it->first.second) and
turnEndpoints_.erase(it->first.second) hence resulting in heap use
after free.

In this patch we change the order of operation so that
servers_.erase(it) is done at the end.

Change-Id: Ie1b87ebc83e39db189ed651690e9e1dce3496b41