Optimize SslHandler in an OpenSslEngine-friendly way

Motivation:

Previous fix for the OpenSslEngine compatibility issue (#2216 and
18b0e95659c057b126653bad2f018a8ce5385255) was to feed SSL records one by
one to OpenSslEngine.unwrap().  It is not optimal because it will result
in more JNI calls.

Modifications:

- Do not feed SSL records one by one.
- Feed as many records as possible up to MAX_ENCRYPTED_PACKET_LENGTH
- Deduplicate MAX_ENCRYPTED_PACKET_LENGTH definitions

Result:

- No allocation of intemediary arrays
- Reduced number of calls to SSLEngine and thus its underlying JNI calls
- A tad bit increase in throughput, probably reverting the tiny drop
  caused by 18b0e95659c057b126653bad2f018a8ce5385255