example: Add support for writing TLS key log file

As NSS and boringssl create the "EXPORTER_SECRET" entry in the key log
file, let's patch OpenSSL too and add keylog callbacks such that these
secrets can be retrieved. The exporter secret can be used with Wireshark
for deriving 1-RTT keys to enable payload decryption.

Fix issues with scan-build (clang)

Server stops accepting unprotected packets after an acknowledgement
for ACK which in turn acknowledges client Finished is received.

Enhance travis build