change RLock to Lock for MutableTree.VersionExists

[yun-yeo]

We monitored, sometimes we met concurrent read write map access panic and found there is some miss-configured lock.

MutableTree.VersionExists has write operation, so we have to change to use write lock.

iavl/mutable_tree.go

Lines 64 to 79 in 577c9a8

	func (tree *MutableTree) VersionExists(version int64) bool {
	tree.mtx.RLock()
	defer tree.mtx.RUnlock()
	if tree.allRootLoaded {
	return tree.versions[version]
	}
	has, ok := tree.versions[version]
	if ok {
	return has
	}
	has, _ = tree.ndb.HasRoot(version)
	tree.versions[version] = has
	return has
	}

[tac0turtle]

thanks for the PR!!

[robert-zaremba]

@odeke-em it turns out that the Mutex improvements are mis-targetted. I'm really curious why we can't use RWMutex. Do you have an idea?

[tac0turtle]

they can be used and are used in places, just can't be used here