Avoid reading the input file twice for InitializableLookupTable in combination with HashTable.

Before this cl, TextFileLineIterator::total_size() was called for HashTable::DoPrepare, even though HashTable::DoPrepare ignores the size parameter.
In order to have a result ready for TextFileLineIterator::total_size(), Init() called GetNumLinesInTextFile(), which read the whole file. Just to throw away the result :-/

This cl:
- adds a DoLazyPrepare, that gets a functor to get the size, only if needed.
- add HashTable::DoLazyPrepare which does not call this functor.
- modify TextFileLineIterator::Init() to not call GetNumLinesInTextFile() anymore, when vocab_size was given as -1.
- modify TextFileLineIterator::total_size() to call GetNumLinesInTextFile() lazily on the first call, if vocab_size_ was passed as -1.

PiperOrigin-RevId: 190593744