segment ebuf can be pre-allocated for better performance

[yihuang]

the number of allocations are clearly dropped, cpu time is also improved marginally.

[semihbkgr]

The size check, determining whether the buffer size exceeds the SegmentSize, is applied after appending the new entry data to the buffer.

var epos bpos
s.ebuf, epos = l.appendEntry(s.ebuf, b.entries[i].index, data)
s.epos = append(s.epos, epos)
if len(s.ebuf) >= l.opts.SegmentSize {
    // segment has reached capacity, cycle now

When the buffer size surpasses its capacity, it re-allocates memory to fit the underlying array.
Therefore, to fully leverage pre-allocation, I think we should ensure that the buffer's capacity is not exceeded.

[yihuang]

the previous behavior allows the segment surpasses its capacity once with the last entry, and we don't have a limit of the size of an entry, it's not easy to preallocate for that I think.