core: make current*Block atomic, and accessor functions mutex-free (#16171)

* core: make current*Block atomic, and accessor functions mutex-free

* core: fix review concerns

* core: fix error in atomic assignment

* core/light: implement atomic getter/setter for headerchain

light/lightchain.go

@@ -171,9 +171,6 @@ func (bc *LightChain) SetHead(head uint64) {
 
 // GasLimit returns the gas limit of the current HEAD block.
 func (self *LightChain) GasLimit() uint64 {
-	self.mu.RLock()
-	defer self.mu.RUnlock()
-
 	return self.hc.CurrentHeader().GasLimit
 }
 
@@ -387,9 +384,6 @@ func (self *LightChain) InsertHeaderChain(chain []*types.Header, checkFreq int)
 // CurrentHeader retrieves the current head header of the canonical chain. The
 // header is retrieved from the HeaderChain's internal cache.
 func (self *LightChain) CurrentHeader() *types.Header {
-	self.mu.RLock()
-	defer self.mu.RUnlock()
-
 	return self.hc.CurrentHeader()
 }