lib-index: Optimize modseq scanning by using the modseq header in index

lib-index: Optimize modseq scanning by using the modseq header in index

View is very commonly set to start reading new changes since dovecot.index
was last written. When reading the newer records in dovecot.index.log, the
modseq of each change is needed to be known. However, the initial modseq
calculation was usually done inefficiently by reading the whole
dovecot.index.log from the beginning of the file. This change prevents
that by using instead the "modseq" header in dovecot.index to get the
initial modseq.