Merge pull request #2 from AlekSi/master

Optimisations to decoder by AlekSi.
Add test for IDs order.
Extract OSMData (PrimitiveBlock) decoder.
Move zlib decompressing.
Preallocate space for zlib decoding.
Return decoded objects.
Do not share decode result.
Make `go vet` happy.
Decode OSMData concurrently.
Check Start error in tests.
Merge pull request #3 from AlekSi/concurrent

Concurrent decoder