This reverts commit b5b4809c.

The commit reverted by this patch made relay advance the consumer
associated with a replica right on subscribe. This is wrong, because the
current implementation of the garbage collector operates on vclock
signatures so that if a replica reconnects with a greater signature than
it had when it last disconnected (e.g. due to replica local changes or
changes pulled from other members of the cluster), the garbage collector
may delete WAL files still needed by the replica, breaking replication.

There are two ways to fix this problem. The first and the most difficult
way is to teach the garbage collector to work with vclocks, i.e. rather
than simply sorting all consumers by signature and using the smallest
signature for garbage collection, maintain a vclock each component of
which is the minimum among corresponding components of all registered
consumers.

The second (easy) way is to advance a consumer only if its acknowledged
vclock is greater than or equal to the vclock of a WAL fed to it. This
way the garbage collector still works with vclock signatures and it's
a responsibility of the caller (i.e. relay) to ensure that consumers are
advanced correctly.

I took on the second way for now, because I couldn't figure out an
efficient way to implement the first. This implies reverting the above
mentioned commit and reopening #4034 - sporadic replication/gc.test.lua
failure - which will have to be fixed some other way.

See the next patch for the rest of the fix and the test.

Needed for #4106