Prevent "index out of bounds" error when sync::SegmentedCache was created with a non-power-of-two segments

[tatsuya6502]

This PR fixes sync::SegmentedCache to create the correct number of internal segments when the specified number of segments is not a power of two (e.g. 5).

When a segmented cache is created with such a number of segments (e.g. 5), the number of the actual internal segments should be rounded up to a nearest power of two (e.g. 8). However the old code only created 5 of segments and caused a panic with "index out of bounds" when writing to or reading from the cache.

let cache = SegmentedCache::new(100, 5); // max 100 entries, 5 segments
assert_eq!(cache.iter().count(), 0);

thread ... panicked at 'index out of bounds: the len is 5 but the index is 5', 
   src/sync/segment.rs:546:10

In addition to fixing the bug, add an unit test sync::segment::tests::non_power_of_two_segments to detect similar bugs in the future.

[tatsuya6502]

Merging.