Implement Equal() for map-valued AttributeValues

[punya]

Description

The implementation AttributeValue.Equals() was incomplete, because it didn't handle values that were of map (kvlist) type. This PR handles that case.

Link to tracking Issue

Possibly #2488.

Testing:

Added to the existing unit test.

[bogdandrutu]

@punya please fix lint errors:

pdata/common.go:290:4: shadow: declaration of "av" shadows declaration at line 263 (govet)
			av := newAttributeValue(&vv[i])
			^
make[2]: *** [../Makefile.Common:26: lint] Error 1

[punya]

@bogdandrutu I'm happy to fix it, but please note that the new declaration as well as the one it shadows are on lines outside of my change.

[bogdandrutu]

@punya not going to fight with the tools :))