wc_db::flush_entries() needs recursion

In svn_wc__db_op_delete(), a call to flush_entries() is made. However, when 
depth==svn_depth_immediates or ==svn_depth_infinity, then we MUST flush entries from all 
subdirectories.

We need to add a depth parameter, and to review all calls to find where the recursive flush needs to 
happen.