Review where we're passing WT_CONNECTION_IMPL instead of WT_SESSION_IMPL

@sueloverso, here are the changes that largely limit passing `WT_CONNECTION_IMPL` to the `__wt_connection_XXX` functions.

I ended up searching `include/extern.h` for functions that took WT_CONNECTION_IMPL as an argument, and searching for all uses of `WT_CONNECTION_IMPL.default_session`, and switching to passing WT_SESSION_IMPL in the cases where I thought it made sense to do so.

Please review and merge if/when you think it's right!