When filtering annotations against CFIs or matching the current segment
against a focus filter, only consider the part of the CFI up to the
first step indirection ("!"). This means that matching only considers whether
CFIs refer to the same content document (which the client also refers to more
generically as a "segment"), and not the precise part of the content document.

This resolves an issue where a CFI focus filter with a step indirection (eg.
"/2/4!/8-/4!/2") did not match the expected segment ("/2").