Bump version to 0.4.0

[guseggert]

No description provided.

[marten-seemann]

We probably want to bundle #172 in this release.

[marten-seemann]

I just merged #172. @guseggert, can you rebase this PR?

[guseggert]

@marten-seemann done!

[github-actions]

Suggested version: v0.4.0
Comparing to: v0.3.0 (diff)

Changes in go.mod file(s):

diff --git a/go.mod b/go.mod
index 40b5748..32ea96f 100644
--- a/go.mod
+++ b/go.mod
@@ -7,9 +7,9 @@ retract v0.2.9 // Contains backwards-incompatible changes. Use v0.3.0 instead.
 require (
 	github.com/gogo/protobuf v1.3.2
 	github.com/hashicorp/golang-lru v0.5.4
-	github.com/ipfs/go-datastore v0.4.5
-	github.com/ipfs/go-ds-badger v0.2.7
-	github.com/ipfs/go-ds-leveldb v0.4.2
+	github.com/ipfs/go-datastore v0.5.0
+	github.com/ipfs/go-ds-badger v0.3.0
+	github.com/ipfs/go-ds-leveldb v0.5.0
 	github.com/ipfs/go-log v1.0.5
 	github.com/libp2p/go-buffer-pool v0.0.2
 	github.com/libp2p/go-libp2p-core v0.8.6

gorelease says:

# github.com/libp2p/go-libp2p-peerstore/pstoreds
## incompatible changes
NewProtoBook: changed from func(github.com/libp2p/go-libp2p-core/peerstore.PeerMetadata) *dsProtoBook to func(github.com/libp2p/go-libp2p-core/peerstore.PeerMetadata, ...ProtoBookOption) (*dsProtoBook, error)
## compatible changes
Options.MaxProtocols: added
ProtoBookOption: added
WithMaxProtocols: added

# github.com/libp2p/go-libp2p-peerstore/pstoremem
## incompatible changes
NewPeerstore: changed from func() *pstoremem to func(...Option) (*pstoremem, error)
NewProtoBook: changed from func() *memoryProtoBook to func(...Option) (*memoryProtoBook, error)
## compatible changes
Option: added
WithMaxProtocols: added

# github.com/libp2p/go-libp2p-peerstore/test
## compatible changes
TestPeerstoreProtoStoreLimits: added

# summary
Suggested version: v0.4.0

gocompat says:

"github.com/libp2p/go-libp2p-peerstore/pstoremem".NewPeerstore SignatureChanged
"github.com/libp2p/go-libp2p-peerstore/pstoremem".NewProtoBook SignatureChanged
"github.com/libp2p/go-libp2p-peerstore/pstoreds".NewProtoBook SignatureChanged