Try to align WritableFileWriter buffered writes

[anand1976]

In buffered IO mode, without checksum calculation for buffered data enabled, try to align writes to the file system on a power of two. This can improve performance, especially on a distributed file system like Warm Storage that does erasure coding and benefits from full stripe writes. We do this by filling up the writable buffer, with a partial append if necessary, before flushing. When checksum calculation for buffered data is enabled, we don't do this since its preferable to not split the data, especially if the caller provides the checksum. We don't guarantee alignment if the caller manually flushes before finishing the file.

Tests:
Add unit tests in file_reader_writer_test and external_sst_file_basic_test.

[pdillinger]

Nice!