Align series to 8/16byte padding to increase addressable space

[gouthamve]

prometheus/prometheus#3190 is because we store the offset to series data in the index and we store in a uint32 giving us only 4GB of addressable space.

To increase it, we can use an uint64 but that would double the postings space required and compression would mean a good chunking mechanism.

Alternately, @fabxc came up with this brilliant idea of aligning the offsets to 8/16bytes and that would mean we just store the multiple of 8/16 bytes, giving us upto 64GB of addressable space.

[shubheksha]

I'll pick this one up!