Fix object lifetime and memory management problems in server code

[murgatroid99]

This includes a few fixes:

• Ensure that client and server credentials construction arguments are null-terminated.
• Ensure that the callback for Server#tryShutdown is always called asynchronously.
• Ensure that the server doesn't get garbage collected while it is running, and remove server destructor code the server down.

[nicolasnoble]

#include <memory>
[...]
std::auto_ptr<StringOrNull> key_strings = new StringOrNull[key_cert_pair_count];
std::auto_ptr<StringOrNull> cert_strings = new StringOrNull[key_cert_pair_count];

And then later use cert_strings[i]->assign().

[nicolasnoble]

Also, from tests:

E0402 15:50:47.195617365   31663 server.cc:1247]             assertion failed: gpr_atm_acq_load(&server->shutdown_flag) || !server->listeners