Introduction of lu_env.

Introduction of lu_env.

With this patch majority of md/dt methods in new server stack get struct lu_env argument instead of struct lu_context.

lu_env consists of two contexts:

 - ->le_ctx: this is "local" context used to avoid allocating data on stack,
 and to avoid short-term dynamic allocations. This is a repalcement for old
 @ctx argument.

 - ->le_ses: this is "session": a context associated with request. Here live
 data that are to be shared between different stack levels. E.g., credentials
 (struct md_cred), and capability.