Open and parse files lazily

[KeiichiHirobe]

Current implementation always parse all of sql files, so may require a lot of time and memory if there are too many files or too big files in directory. But, in most of cases, we only need to parse 1~3 files because sort byId does not need content of files, it needs only the name of files. So, I think it is better to parse sql files only when we need content of files.

Some migration sources are not supported to open/parse lazily in this PR. HttpFileSystemMigrationSource,FileMigrationSource and EmbedFileSystemMigrationSource are supported, MemoryMigrationSource,AssetMigrationSource and PackrBox are not supported.

I don't think that's a problem because I guess most users don't use the latter migration sources.

I believe lazy loading should be the default behavior, but I made it optional with SetLazyLoad(true) to keep backward compatibility.

ref: https://github.com/rubenv/sql-migrate/issues/203