ovsdb-server: Fix memory leak using perf counter without initialization.

ovsdb-server: Fix memory leak using perf counter without initialization.

perf_counter_accumulate() is invoked without perf_counters_init() being
called first, which leads to a memory leak reported by Valgrind (test
cases 104, 106, and 107). A call trace is below:
    xmalloc (util.c:112)
    shash_add_nocopy__ (shash.c:109)
    shash_add_nocopy (shash.c:121)
    shash_add (shash.c:129)
    shash_add_once (shash.c:136)
    shash_add_assert (shash.c:146)
    perf_counter_init (perf-counter.c:86)
    perf_counter_accumulate (perf-counter.c:95)
    ovsdb_txn_commit (transaction.c:850)
    ovsdb_file_open__ (file.c:217)
    open_db (ovsdb-server.c:418)
    main (ovsdb-server.c:263)

Signed-off-by: William Tu <u9012063@gmail.com>
Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>
Co-authored-by: Daniele Di Proietto <diproiettod@vmware.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>