This commit addresses minor bugs introduced with commit
6faaa91:

- The number of threads used by the database engine was
  incorrectly clamped to the lower possible value, such
  that the database was effectively operating in single
  threaded mode.

- The number of requests to extract at once was so high
  that it could result in increased latency. The bundle
  size is now limited to 4 and can be adjusted by a new
  configuration option `rq_bundle` in the `[node_db]`
  stanza. This is an advanced tunable and adjusting it
  should not be needed.

Several hard-coded parameters control the behavior of the ledger
acquisition engine. The values of many of these parameters where
set by intuition and have complex and non-intuitive interactions
with each other and other parts of the code.

An earlier commit attempted to adjust several of these parameters
to improve syncing performance; initial testing was promising but
a number of operators reported experiencing syncing and stability
issues with their servers. As a result, this commit reverts parts
of commit 1823506.

This commit further adjusts some tunables so as to increase the
aggressiveness of the ledger acquisition engine.

failing online delete healch check.

Fixes XRPLF#3996.

* "A path is considered invalid if and only if it enters and exits an
  address node through trust lines where No Ripple has been enabled for
  that address." (https://xrpl.org/rippling.html#specifics)
* When loading trust lines for an account "Alice" which was reached
  via a trust line that has the No Ripple flag set on Alice's side, do
  not use or cache any of Alice's trust lines which have the No Ripple
  flag set on Alice's side. For typical "end-user" accounts, this will
  return no trust lines.

* Also remove an unused variable that's giving VS a problem