We introduce a shard_param object to hold the shard director lookup
parameters which until now could only be passed to the .backend()
method.

By associating a parameter object with a shard director, we enable
LAZY lookups as with the other directors. Parameter objects are
defined with VCL scope (normal vmod objects), but can be overridden
per backend request using a task priv.

We use the same concept to carry shard.backend() parameters to
vdi resolve for LAZY mode: They get saved in a per-director task
scope parameter object.

Each object points to another object providing defaults for
values which are not defined.

Actual resolution of the various parameter objects does not happen
before they are used to allow changing them independently (ie,
shard .backend() parameters have precedence over an associated
parameter object, which by itself can be overridden).

Overview of parameter objects (pointers are alternatives)

shard() director	shard_param() object	default praram

	     --------------------------------->	  vmod static
  VCL obj   /				     ->
  .param  -+--------->	  VCL obj	    /  _
  			  .default  --------   /|
					      /
			     ^		     /
			     |		    /
					   /
			  .default	  /
	------------->	  TASK priv	 /
       /				/
  .default -----------------------------
  TASK priv