Optimize marshalled size of web sessions

There are a couple of marshalling optimizations that we can make in the current web session clustering code:

• We currently marshal session identifiers as UTF-8 strings. For the default session id length, this result in 42 bytes for each cache key. Since session ids use a limited alphabet (modified base64) we can easily reduce this to 30 bytes (31 to support variable length).
• We currently store the metadata of a session in a single cache entry. The metadata is composed of:
  1. Creation timestamp (long)
  2. Last access timestamp (long)
  3. Max inactive interval (int)

#1 is static. #2 updates every request. #3 is "effectively" static. If we split the metadata cache entry into two: one containing static metadata, the other dynamic metadata; we can reduce the size of the data that needs to be replicated for a read-only request. Additionally, #2 can be represented as a duration of time since #1, and thus can be expressed as a short or integer instead of a long. Thus, for requests that do not modify the session, we only need to replicate 4 bytes or less instead of 20.