Make the sweep_interval individually configurable:

The "sweep interval" is the amount of time between successive sweeps of
of various in-memory data structures to remove stale items.

Prior to this commit, the interval was automatically adjusted, based on
the value of the `[node_size]` option in a server's configuration file.

If merged, this commit introduces a new configuration option that makes
it possible for a server operator to adjust the sweep interval and make
a CPU/memory tradeoff:

    [sweep_interval]
    <integer>

The specified value represents the number of seconds between successive
sweeps. The range of valid values is between 10 and 600.

Important operator notes:

This is an advanced configuration option that should not be used unless
there is empirical data which suggests that the default sweep frequency
is either resulting in performance problems or is causing undue load to
the server.

Note that adjusting the sweep interval may not have the intended effect
on the server. Lower values will not always translate to a reduction of
memory usage and higher values will not always translate to a reduction
of CPU usage and/or load.

src/ripple/core/impl/Config.cpp

@@ -627,6 +627,15 @@ Config::loadFromString(std::string const& fileContents)
     if (getSingleSection(secConfig, SECTION_DEBUG_LOGFILE, strTemp, j_))
         DEBUG_LOGFILE = strTemp;
 
+    if (getSingleSection(secConfig, SECTION_SWEEP_INTERVAL, strTemp, j_))
+    {
+        SWEEP_INTERVAL = beast::lexicalCastThrow<std::size_t>(strTemp);
+
+        if (SWEEP_INTERVAL < 10 || SWEEP_INTERVAL > 600)
+            Throw<std::runtime_error>("Invalid " SECTION_SWEEP_INTERVAL
+                                      ": must be between 10 and 600 inclusive");
+    }
+
     if (getSingleSection(secConfig, SECTION_WORKERS, strTemp, j_))
         WORKERS = beast::lexicalCastThrow<std::size_t>(strTemp);