Reimplement DB collections for mirrors, repos and snapshots

See #765, #761

Collections were relying on keeping in-memory list of all the objects
for any kind of operation which doesn't scale well the number of
objects in the database.

With this rewrite, objects are loaded only on demand which might
be pessimization in some edge cases but should improve performance
and memory footprint signifcantly.

Makefile

@@ -13,7 +13,7 @@ RUN_LONG_TESTS?=yes
 
 GO_1_10_AND_HIGHER=$(shell (printf '%s\n' go1.10 $(GOVERSION) | sort -cV >/dev/null 2>&1) && echo "yes")
 
-all: test check system-test
+all: test bench check system-test
 
 prepare:
 	go get -u github.com/alecthomas/gometalinter
@@ -57,6 +57,9 @@ else
 	go test -v `go list ./... | grep -v vendor/` -gocheck.v=true
 endif
 
+bench:
+	go test -v ./deb -run=nothing -bench=. -benchmem 
+
 mem.png: mem.dat mem.gp
 	gnuplot mem.gp
 	open mem.png