Add documentation for ledger entries (RIPD-361)
Impose a local limit on path lengths (RIPD-350)
Find 'sabfd' paths (RIPD-335)
This permits USD/GW1 to be bridged to USD/GW2 by BTC/GW3 or
USD/GW1 to be bridged to BTC/GW2 by CNY/GW3.
Fix account_tx resume (RIPD-314)
Various tidying:
* Fix unused variable warnings.
* Clean unused items from TER.
* Improvement to LES use of shared_ptr.
Pass and use time_point in DecayingSample ctor (RIPD-359)
Switches a number of places in Resource::Logic to use abstract_clock::now()
which returns a time_point.  Unfortunately Resource::Logic tracks time
locally also, but with ints, not time_point.  So Resource::Logic uses a
delicate mix of abstract_clock::now() and abstract_clock::elapsed() with
this commit.  That inconsistency could be addressed in a second commit.
Serialization code improvements:
* Add generics to eliminate duplicate code in STHash
* Use generics in Serializer and callers