Do not use FileChannel.position() in FileChannelJournalSegmentWriter

Description

FileChannelJournalSegmentWriter is using FileChannel methods in sub-optimal way, such as:

// Read more bytes from the segment if necessary.
        if (memory.remaining() < maxEntrySize) {
          channel.position(position);
          memory.clear();
          channel.read(memory);
          channel.position(position);
          memory.flip();
        }

and

// Zero entries after the given index.
        long position = channel.position();
        channel.write(zero());
        channel.position(position);

This incurs syscall overhead and is bad for reasoning as highlighted in CONTROLLER-2094. This manifests as an explicit lseek(SEEK_CUR) at each write, as shown in https://lf-opendaylight.atlassian.net/browse/CONTROLLER-2043#icft=CONTROLLER-2043:

1322466 lseek(109, 0, SEEK_CUR)         = 1435610 <0.000009>
1322466 write(109, "\0\1\220\203aE3\367\22\1\200uui\344\254\355\0\5sr\0Corg.opend"..., 102539) = 102539 <0.000033>
1322466 fsync(109)

Reorganize the code to use a field to store the current channel position and use FileChannel.write(ByteBuffer, long) to issue a pwrite(2).

This should eliminate one syscall and reduce locking around FileChannel.positionLock.