Fix incorrect address in connectivity check report:
The remoteAddress is the address as seen on the socket, which for
incoming connections has a random port chosen by the remote implementation
that is different from the port number used to accept connections by the
remote listening socket. The checkedAddress is the remote address as seen
on the socket, combined with the port advertised in the TMEndpoints message.
This fixes the reporting and metadata associated with addresses tested
for connectivity.

The README has been updated to reflect that uptime is no longer part of
the metadata associated with IP addresses saved for bootstrapping.
Cleanup: Combine Section and BasicConfig, move to basics
Remove unused file
Add missing include in beast header
Add pending_handlers
Clean up Overlay and PeerFinder sources:
* Tidy up identifiers and declarations
* Merge PeerFinder headers into one file
* Merge handout classes and functions into one file
Refactor PeerFinder:
Previously, the PeerFinder manager constructed with a Callback object
provided by the owner which was used to perform operations like connecting,
disconnecting, and sending messages. This made it difficult to change the
overlay code because a single call into the PeerFinder could cause both
OverlayImpl and PeerImp to be re-entered one or more times, sometimes while
holding a recursive mutex. This change eliminates the callback by changing
PeerFinder functions to return values indicating the action the caller should
take.

As a result of this change the PeerFinder no longer needs its own dedicated
thread. OverlayImpl is changed to call into PeerFinder on a timer to perform
periodic activities. Furthermore the Checker class used to perform connectivity
checks has been refactored. It no longer uses an abstract base class, in order
to not type-erase the handler passed to async_connect (ensuring compatibility
with coroutines). To allow unit tests that don't need a network, the Logic
class is now templated on the Checker type. Currently the Manager provides its
own io_service. However, this can easily be changed so that the io_service is
provided upon construction.

Summary
* Remove unused SiteFiles dependency injection
* Remove Callback and update signatures for public APIs
* Remove obsolete functions
* Move timer to overlay
* Steps toward a shared io_service
* Templated, simplified Checker
* Tidy up Checker declaration
Load PeerFinder database in Stoppable::onPrepare:
OverlayImpl::onStart calls into PeerFinder before PeerFinder::Manager::onStart,
causing tests to sometimes fail and the application to intermittently not start.
The order of calls to Stoppable::onStart is implementation defined and not
predictable.

This changes PeerFinder to load the database in Stoppable::onPrepare, before
threads are launched. In general, creation and initialization of resources that
are shared between classes should happen in onPrepare rather than onStart,
to solve this problem.
SHAMap performance improvements (RIPD-434)

This reworks the way SHAMaps are stored, flushed, backed, and
traversed. Rather than storing the linkages in the SHAMap itself,
that information is now stored in the nodes. This makes
snapshotting much cheaper and also allows traverse work done on
behalf of one SHAMap to be used by other SHAMaps that share inner
nodes with that SHAMap.

When a SHAMap is modified, nodes are modified all the way up to the
root. This means that the modified nodes in a SHAMap can easily be
traversed for flushing. So they don't need to be separately tracked.

Summary
* Remove mTNByID
* Remove mDirtyNodes
* Much faster traverses
* Much Faster snapshots
* New algorithm for flushing
* New vistNodes/visitLeaves
* Avoid I/O if a map is unbacked