common/bloom_filter: fix estimated element count for compressed filters

We need to compensate for the fact that there are fewer bits than there
used to be.

This is a crappy adjustment; it is non-linear.  It's clone enough for now,
though.

Signed-off-by: Sage Weil <sage@inktank.com>
common/bloom_filter: fix compress; improve argument

Fix bug in compress() when compressing to less than .5 or original.

Make the argument have sane units (target size relative to current size;
not a precentage reduction).

Signed-off-by: Sage Weil <sage@inktank.com>
common/bloom_filter: simplify compressible_bloom_filter encode/decode/dump

Re-use bloom_filter for this, as per Loic's suggestion.  Much nicer!

Signed-off-by: Sage Weil <sage@inktank.com>
common/bloom_filter: add a few assertions and checks for bit_table_ == NULL

Signed-off-by: Sage Weil <sage@inktank.com>