HRMP: Constrain the maximum number of HRMP messages per candidate

This commit addresses the HRMP part of #1869

roadmap/implementers-guide/src/runtime/router.md

@@ -189,6 +189,7 @@ Candidate Acceptance Function:
         1. equal to the context's block number
         1. or in `HrmpChannelDigests` for `P` an entry with the block number should exist
 * `check_outbound_hrmp(sender: ParaId, Vec<OutboundHrmpMessage>)`:
+    1. Checks that there are at most `config.hrmp_max_message_num_per_candidate` messages.
     1. Checks that horizontal messages are sorted by ascending recipient ParaId and there is no two horizontal messages have the same recipient.
     1. For each horizontal message `M` with the channel `C` identified by `(sender, M.recipient)` check:
         1. exists

roadmap/implementers-guide/src/types/runtime.md

@@ -85,10 +85,16 @@ struct HostConfiguration {
 	/// The maximum number of inbound HRMP channels a parathread is allowed to accept.
 	pub hrmp_max_parathread_inbound_channels: u32,
 	/// The maximum size of a message that could ever be put into an HRMP channel.
+	///
+	/// This parameter affects the upper bound of size of `CandidateCommitments`.
 	pub hrmp_channel_max_message_size: u32,
 	/// The maximum number of outbound HRMP channels a parachain is allowed to open.
 	pub hrmp_max_parachain_outbound_channels: u32,
 	/// The maximum number of outbound HRMP channels a parathread is allowed to open.
 	pub hrmp_max_parathread_outbound_channels: u32,
+	/// The maximum number of outbound HRMP messages can be sent by a candidate.
+	///
+	/// This parameter affects the upper bound of size of `CandidateCommitments`.
+	pub hrmp_max_message_num_per_candidate: u32,
 }
 ```