Read override file flags from file only once for config server

Up until now, every lookup of a flag in the ConfigServerFlagSource would
 1. try to read 2 flag files under /etc/vespa/flags, causing exceptions
    because they are typically not set, and
 2. then read flag from ZooKeeper through ZooKeeperFlagSource

Optimization was deliberately held off until later (now). This PR fixes (1).

Changes the ConfigServerFlagSource to:
 1'. Read VESPA_HOME/var/vespa/flag.db once during component graph
     construction.  As before, if a flag is defined on file, the flag is not
     looked up in ZK, which may be useful in emergencies.
 2.  As before.

Also, removes the last usages of FileFlagSource and its reading of flags in
/etc/vespa/flags.

0 files changed, 0 insertions, 0 deletions