Standard ygot.Diff API flattens both structs into {path, value} pair
list and compares them to resolve new, modified and deleted values.
This approach does not scale well for large ygot trees. Following are
the go benchmarks for ygot.Diff API with openconfig-acl ygot trees.
Size "MxN" stands for ygot tree with M ACLs and N rules in each ACL.

+-------------------+---------------+----------------+
|   Test case       |  size=10x10   |  size=10x100   |
+-------------------+---------------+----------------+
| No changes        | 9024089697ns  | 854023452896ns |
| One attr changed  | 9285911808ns  | 857292949475ns |
| All attrs changed | 9333680743ns  | 855581319202ns |
| Create all attrs  |   80830018ns  |    858752644ns |
| Delete all attrs  |   83082154ns  |    755762682ns |
+-------------------+---------------+----------------+

Implemented a custom ygot diff tool to compare the ygot struct fields
inline. Uses reflection to traverse and compare structs.
Function signature:

func Diff(s1, s2 ygot.GoStruct, opt DiffOptions) (DiffResults, error)

type DiffOptions struct {
    RecordAll bool // include all s2 attrs, even if not modified
}

type DiffResults struct {
    Update []*gnmi.Update
    Delete []*gnmi.Path
}

Diff function will be used for generating notification messages for
SMPLE subscriptions -- by comparing ygot objects from the previous
iteration with the current objects. By default, Diff returns delete
paths and update values for new/modified fields only. Suitable for
SAMPLE subscription processing when suppress_redundant=true. The
suppress_redundant=false case requires all set attributes of the ygot
struct as update values (even if they are not modified). This can be
achieved by passing RecordAll=true in the DiffOptions.

Go benchmark for the new Diff API with openconfig-acl model:
+-------------------+------------+-------------+---------------+
|   Test case       | size=10x10 | size=10x100 | size=100x1000 |
+-------------------+---------------+--------------------------+
| No changes        | 1784778ns  | 15460520ns  | 1757874254ns  |
| One attr changed  | 1547512ns  | 16506496ns  | 1786245108ns  |
| All attrs changed | 3562498ns  | 34306626ns  | 3846524858ns  |
| Create all attrs  | 7101530ns  | 74663888ns  | 8245685970ns  |
| Delete all attrs  |  120232ns  |   125293ns  |     453306ns  |
+-------------------+------------+-------------+---------------+

Signed-off-by: Sachin Holla <sachin.holla@broadcom.com>