Update boltdb dependency to fix golang 1.7 intermittent failures

We have a old boltdb/bolt, etcd has moved to newer boltdb, so we should
do the same. Specifically this change needs to be in our tree:
92410e0673

as this fixes intermittent issues we see in our CI runs. So in this
PR, we vendor the v1.3.0 version of boltdb/bolt.

Fixes #43973

vendor/github.com/boltdb/bolt/page.go

@@ -62,6 +62,9 @@ func (p *page) leafPageElement(index uint16) *leafPageElement {
 
 // leafPageElements retrieves a list of leaf nodes.
 func (p *page) leafPageElements() []leafPageElement {
+	if p.count == 0 {
+		return nil
+	}
 	return ((*[0x7FFFFFF]leafPageElement)(unsafe.Pointer(&p.ptr)))[:]
 }
 
@@ -72,6 +75,9 @@ func (p *page) branchPageElement(index uint16) *branchPageElement {
 
 // branchPageElements retrieves a list of branch nodes.
 func (p *page) branchPageElements() []branchPageElement {
+	if p.count == 0 {
+		return nil
+	}
 	return ((*[0x7FFFFFF]branchPageElement)(unsafe.Pointer(&p.ptr)))[:]
 }