Fix a bug in the shared cache implementation.

We could decrement an unsigned number into negative. Also add
a test case and some better documentation to the source code.
The WT_NOTFOUND return should work the same as other non-zero returns and leave cursors pointing to the original key/value pair.

refs #1209
api/leveldb/leveldb_wt_config.in: copyright information is incorrect
Have test/format always reset cursors at the end of each operation.
It turns out that LSM depended on a `WT_NOTFOUND` return clearing `WT_CURSTD_KEY_SET`.  Change LSM for now to keep test/format running, but we should review the way that LSM tracks whether chunk cursors are positioned.
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