Update shared cache configuration options:

* Change enable to enabled to match log setting.
* Return an error if there is a shared_cache configuration string
  but it's not enabled and not explicit cache_size configuration.
Add upgrade doc for shared_cache config changes.

Also fine tune when we return an error in shared cache configuration parsing.
Update shared cache test to match configuration changes.
typo
Update sharec_cache configuration again.

Remove enabled option, set default value for name to empty, and require
that a name is provided if any shared_cache configuration is provided.

Also update upgrading documentation and tests.
Simplify page walk: all the cases end up doing the same page-swap operation.
If we underflow a page size during a merge, that should not be fatal: just limit the memory size transferred to the original page size.
Merge pull request #906 from wiredtiger/shared-cache-config

Shared cache config
When searching for pages to merge, don't walk up into a locked page: that can race with eviction.

refs #909