11684: Instead of fiddling with ArvadosFile object's internals from the BlockManager

11684: Instead of fiddling with ArvadosFile object's internals from the BlockManager
put threads to update the segments locators when committing synchronously a block
built from smaller blocks, take advantage of ArvadosFile.flush() existing mechanism
to update unrealized segments locators by building a list of bufferblock owners
and calling all owner's flush() method on commit_all().
To avoid calling delete_bufferblock() many times on a single bufferblock, added
a flag on flush() and delete the bufferblock after flushing all owners.