Track skipped transactions

Track skipped transactions

We are allocating transaction identifiers which are shared across
shards. As a transaction can touch only some shards, non-participating
shards will never see the transaction ID, leading to holes in their
UnsignedLongSets -- and those holes gradually eat up more and more
memory.

Track when we have such a hole and lazily forward a new request, which
purges one or more such identifiers -- plugging the memory leak.

The crux of the machinery is exchanging state between ProxyHistory
and FrontendHistoryMetadataBuilder.

JIRA: CONTROLLER-1991
Change-Id: I3817fa2841e5f9c405bb20ff1a104537ad459ce3
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>