Suggest using smaller key size for cache2

rfc 5246 (TLS 1.2) doesn't put any limit on session ID length, but openssl docs mentions 32 bytes for SSLv3/TLS, so this may be implementation specific. In practice I see that session IDs are 64 chars long so I think that suggesting using 128 byte limit is safe.

This may bring significant memory gains:

``cache2 = name=ssl,items=10000,keysize=128,blocksize=4096`` = 40MB
``cache2 = name=ssl,items=10000,blocksize=4096`` = 59MB