feat: fix mempool sync start early

[hansieodendaal]

Description

Mempool sync can only start when is_synced is set to true.

• An edge case was witnessed whereby mempool sync can start early if the entire network is generally
  slow to respond to pings. The ChainMetadataEvent::NetworkSilence event is transmitted if 3x
  consecutive ping cycles were started without corresponding pongs received, which sets
  is_synced to true. The moment that peer metadata is then received, block sync and mempool sync
  will occur at the same time.
• Prolonged network silence is now detected in the Starting state. At startup, the first successful metadata message received from a peer will signal the transition to Listening, otherwise Listening will be notified of the network silence, which can act accordingly.
• Additional debug logs were added to track all events that can set the is_synced to true.
• Removed log-mdc from the dependencies as the relevant code is not used.

The edge case warning message must be monitored to see if it is a real issue:

warn!(
    target: LOG_TARGET,
    "Initial sync achieved based on event 'NetworkSilence'; this may not be true if the entire \
    network in general is slow to respond to pings"
);

Fixes #6766

Motivation and Context

Mempool sync "started" before block sync was achieved.

How Has This Been Tested?

System-level testing

What process can a PR reviewer use to test or verify this change?

Code review
System-level testing

Breaking Changes

• None
• Requires data directory on base node to be deleted
• Requires hard fork
• Other - Please specify

[github-actions]

Test Results (CI)

    3 files    129 suites   37m 13s ⏱️
1 352 tests 1 352 ✅ 0 💤 0 ❌
4 054 runs  4 054 ✅ 0 💤 0 ❌

Results for commit 45212cc.

♻️ This comment has been updated with latest results.

[github-actions]

Test Results (Integration tests)

36 tests   36 ✅  22m 52s ⏱️
11 suites   0 💤
 2 files     0 ❌

Results for commit 17a6d70.

♻️ This comment has been updated with latest results.