Add __whence args to fd_func_enter() and fd_func_exit() so that their log
messages get reported from the location of the IN() or OUT() macro, not a line
in performance_timing.c.
Removed diagnostic information from the FATAL() message in fd_func_exit(), and
a comment explaining why (causes SEGV).
Replace the main-loop scheduled periodic alarm with an "activate" alarm that is
scheduled whenever a fetch candidate is added to any queue, unless the alarm is
already scheduled.
Replace the "rhizome.fetch_interval_ms" config item with
"rhizome.fetch_delay_ms" [default 50], which is the number of milliseconds
between adding a fetch candidate and firing the "activate" alarm. This allows
time for a few more Rhizome advertisment packets to arrive after the first one,
before deciding which fetches to start first.
Add new `is_scheduled()` alarm primitive.
SET_NONBLOCKING(), SET_BLOCKING(), WRITE_STR() are now set_nonblock(),
set_block() and write_str() respectively, all of which log an error before
returning -1. There are other useful methods: write_all() treats anything less
than all bytes written as an error; write_nonblock() treats EAGAIN and EINTR as
zero bytes written, and a combination: write_all_nonblock().
