Use exp format and more precision output for ZSCAN.

Ref: issue #2175
Handle infinite explicitly in createStringObjectFromLongLong().
Regression test for issue #2175.
Don't show the ASCII logo if syslog is enabled.

Closes issue #1935.
Network bandwidth tracking + refactoring.

Track bandwidth used by clients and replication (but diskless
replication is not tracked since the actual transfer happens in the
child process).

This includes a refactoring that makes tracking new instantaneous
metrics simpler.