SwiftNIO SSL 2.1.0

Semver Minor

• Add a timeout to TLS shutdown, ensuring that channels don't get stuck open. (#100)
• Add a callback that enables SSLKEYLOGFILE support. (#98)

Semver Patch

• Correctly set SSL_VERIFY_FAIL_IF_NO_PEER_CERT when turning on cert verification. (#107)
• Fix an issue where we use non-thread-safe buffers to print error strings. (#102)
• Update BoringSSL. (#108)
• Improve API docs. (#103)