Reduce internal strucutre size in memory

[ikopylov]

There are several places where we can reduce the size of internal structs. Since there are expected to be about 100k instances of Storage in production, even a small reduction can give good improvements in memory consumption.

1. Bloom filter config sharing. It is expected, that all instances will have the same config. This shared value can be stored in the static field with lazy initialization (https://github.com/qoollo/pearl/blob/master/src/filter/bloom.rs#L16)
2. FileName consumes too much memory (https://github.com/qoollo/pearl/blob/master/src/blob/core.rs#L466)