shards: a progress message every 10s when loading
shards: a progress message every 10s when loading

A zoekt-webserver does not start listening until it has loaded all
shards. In some cases (eg misconfigured IO) the startup time can be very
large. This commit adds a progress message so admins can understand what
is going on.

Change-Id: I997543ce4204f1de48f4071f635668b70c299bb4

google/zoekt,google/zoekt