chore(server): rdb save can now save into tcp socket directly. (#317)

In more detail, RdbSaver uses AlignedBuffer that writes into io::Sink in chunks of 4KB.
It's great for the direct file I/O, but bad for sockets that receive blocks of 4KB with garbage
at the end. I improved the code around this and actually simplified the logic, so now AlignedBuffer
is just another Sink that is passed into serializer when writing into files. When sending to
sockets a socket sink is passed instead.

Also many other unrelated changes grouped into this pretty big cr.
1. dashtable readability improvements.
2. Move methods from facade::ConnectionContext - into facade::Service,
   make ConnectionContext a dumb object.
3. Optionally allow journal to be memory only (not backed up by a disk)
   by using a ring buffer to store last k entries in each journal slice. Also renamed
   journal_shard into journal_slice because journal has presence in each DF thread and not
   only in its shards.
4. Introduce journal::Entry that will consolidate any store change that happens in the thread.
5. Introduce GetRandomHex utility function.
6. Introduce two hooks: ServerFamily::OnClose that is called when a connection is closed,
   and ServerFamily::BreakOnShutdown that is called when process exits and any background fibers neet to
   break early.
7. Pull some noisy info logs out of rdb_load class.
8. Snapshot class now has the ability to subscribe to journal changes, thus it can include concurrent changes into the snapshot.
   Currently only journal::Op::VAL is supported (it's part of RDB format anyway).

Signed-off-by: Roman Gershman <roman@dragonflydb.io>

src/server/transaction.cc

@@ -1160,7 +1160,7 @@ bool Transaction::NotifySuspended(TxId committed_txid, ShardId sid) {
   return false;
 }
 
-void Transaction::BreakOnClose() {
+void Transaction::BreakOnShutdown() {
   if (coordinator_state_ & COORD_BLOCKED) {
     coordinator_state_ |= COORD_CANCELLED;
     blocking_ec_.notify();