Replace static buf with palloc in str_time()

Replace static buf with palloc in str_time()

The function is used only once in the startup process, so the leak
into current memory context is harmless.

This is a tiny step in making the server thread-safe.

Reviewed-by: Robert Haas
Discussion: https://www.postgresql.org/message-id/7f86e06a-98c5-4ce3-8ec9-3885c8de0358@iki.fi