Limit file growth rate#56

At the moment, lgalloc allocates files per size class with doubling capacity. This minimizes the number of files, but can cause contention on the kernel data structures: each file is mapped in its entirety. Linux manages the mapping as a single virtual mapping (VMA), and some operations require locks, which can serialize access to the mappings. This shows as the background worker fencing out other threads during maintenance, which is undesirable as it causes latency spikes and dropped throughput for other threads. Instead, lgalloc could do two things: * Grow files with a smaller exponent instead of doubling, for example 1.3, and make it configurable. * Grow files in doubling capacity, but map multiple times in non-overlapping chunks.