Create a new block manager when writing files

[drdavella]

Currently the same block manager is used for all writes of the same AsdfFile object, and it retains state between calls to write_to. This is not good because it leads to confusing and sometimes erroneous behavior with respect to the storage type of certain arrays (see #599 and #618 for one particular example).

When creating an AsdfFile object with the intent to write a new file, there is no need to create a block manager until actually calling write_to. This should help resolve ambiguities without introducing any other user-visible effects.