Test for multi-field ops + minor corrections.

The new test runs 100k per 100k transactions which call Modify, each 100k
on an increasing number of independent Shielded<int> fields. Results on my
laptop (Mono 2.10, Ubuntu 13.10, i5-2430M 2.4 GHz, 4 GB RAM) show that
a signle field trans takes a little under 3 microseconds, and that each new field
linearly increases it's running time by ~0.7 microseconds.

Also included:
* A minor change in CommitSubscription, a missing "readonly".
* A minor correction in Shield.TrimCopies, in the condition determining whether
a set of copies can be trimmed. It was a bit too conservative.