Add txrequest module

This adds a new module (unused for now) which defines TxRequestTracker, a data
structure that maintains all information about transaction requests, and coordinates
requests.

src/Makefile.am

@@ -215,6 +215,7 @@ BITCOIN_CORE_H = \
   timedata.h \
   torcontrol.h \
   txdb.h \
+  txrequest.h \
   txmempool.h \
   undo.h \
   util/asmap.h \
@@ -327,6 +328,7 @@ libbitcoin_server_a_SOURCES = \
   timedata.cpp \
   torcontrol.cpp \
   txdb.cpp \
+  txrequest.cpp \
   txmempool.cpp \
   validation.cpp \
   validationinterface.cpp \