A reusable object that will calculate, given a list of PeerFilterProviders, a merged
BloomFilter and earliest key time for all of them.
Used by the PeerGroup class internally.
Thread safety: threading here can be complicated. Each filter provider is given a begin event, which may acquire
a lock (and is guaranteed to receive an end event). This class is mostly thread unsafe and is meant to be used from a
single thread only, PeerGroup ensures this by only accessing it from the dedicated PeerGroup thread. PeerGroup does
not hold any locks whilst this object is used, relying on the single thread to prevent multiple filters being
calculated in parallel, thus a filter provider can do things like make blocking calls into PeerGroup from a separate
thread. However the bloomFilterFPRate property IS thread safe, for convenience.