support wasm

[viteshan]

go-vite fails to compile to wasm.

Probably related to syndtr/goleveldb#288.

go1.15.15

GOARCH=wasm GOOS=js go build cmd/gvite/main.go

Expected behaviour
compile to wasm

Actual behaviour
some package does not compile to wasm

# github.com/vitelabs/go-vite/v2/cmd/utils/flock
cmd/utils/flock/flock.go:26:11: undefined: newLock
# github.com/syndtr/goleveldb/leveldb/storage
../../../../pkg/mod/github.com/syndtr/goleveldb@v1.0.1-0.20210305035536-64b5b1c73954/leveldb/storage/file_storage.go:107:16: undefined: newFileLock
../../../../pkg/mod/github.com/syndtr/goleveldb@v1.0.1-0.20210305035536-64b5b1c73954/leveldb/storage/file_storage.go:192:3: undefined: rename
../../../../pkg/mod/github.com/syndtr/goleveldb@v1.0.1-0.20210305035536-64b5b1c73954/leveldb/storage/file_storage.go:267:12: undefined: rename
../../../../pkg/mod/github.com/syndtr/goleveldb@v1.0.1-0.20210305035536-64b5b1c73954/leveldb/storage/file_storage.go:272:12: undefined: syncDir
../../../../pkg/mod/github.com/syndtr/goleveldb@v1.0.1-0.20210305035536-64b5b1c73954/leveldb/storage/file_storage.go:555:9: undefined: rename
../../../../pkg/mod/github.com/syndtr/goleveldb@v1.0.1-0.20210305035536-64b5b1c73954/leveldb/storage/file_storage.go:591:13: undefined: syncDir
# github.com/vitelabs/go-vite/v2/common/db/xleveldb/storage
common/db/xleveldb/storage/file_storage.go:81:16: undefined: newFileLock
common/db/xleveldb/storage/file_storage.go:166:3: undefined: rename
common/db/xleveldb/storage/file_storage.go:253:11: undefined: rename
common/db/xleveldb/storage/file_storage.go:258:11: undefined: syncDir
common/db/xleveldb/storage/file_storage.go:355:14: undefined: rename
common/db/xleveldb/storage/file_storage.go:484:9: undefined: rename
common/db/xleveldb/storage/file_storage.go:520:13: undefined: syncDir