CreateTrigger and DropTrigger

[kelindar]

This PR adds trigger functionality for the cases where built-in capabilities aren't sufficient. Triggers allow you to add a callback function that will get called whether a value is inserted, updated or deleted. It functions similarly to the bitmap index.

players.CreateTrigger("on_balance", "balance", func(r Reader) {
	switch {
	case r.IsDelete():
		updates = append(updates, fmt.Sprintf("delete %d", r.Index()))
	case r.IsUpsert():
		updates = append(updates, fmt.Sprintf("upsert %d=%v", r.Index(), r.Float()))
	}
})

// Perform a few deletions and insertions
for i := 0; i < 3; i++ {
	players.DeleteAt(uint32(i))
	players.Insert(func(r Row) error {
		r.SetFloat64("balance", 50.0)
		return nil
	})
}

// Must keep track of all operations
assert.Len(t, updates, 6)
assert.Equal(t, []string{
	"delete 0", 
	"upsert 500=50", 
	"delete 1", 
	"upsert 501=50", 
	"delete 2",
	"upsert 502=50",
}, updates)