Add support for custom election tick

[LakshimiRam-073]

Description

This PR adds election-tick as a configurable sub-option to the existing --raft superflag for both Alpha and Zero nodes.

Currently, ElectionTick is hardcoded to 20 in conn/node.go:91 (equivalent to 2 seconds at the 100ms tick interval). This makes it impossible for operators to control leader election priority across nodes deployed in heterogeneous network topologies -- for example, nodes spread across different network segments with varying inter-node RTTs (1-2ms within the same segment vs. 10-50ms across segments), or nodes behind different network switches with asymmetric bandwidth, latency, and jitter characteristics.

Changes:

1. worker/server_state.go — Added election-tick=20 to RaftDefaults string (preserves backward compatibility)
2. conn/node.go — Read election-tick from x.WorkerConfig.Raft config instead of hardcoded value
3. Validation — Added minimum bound check ensuring election-tick >= 10 * HeartbeatTick per etcd/raft library requirements

Use case: In deployments where Alpha nodes span different network segments with asymmetric latency, operators can now bias leadership toward low-latency nodes:

# Low-latency segment — fast election timeout (1-2s)
dgraph alpha --raft="idx=1; group=1; election-tick=10"

# High-latency segment — conservative timeout (5-10s)
dgraph alpha --raft="idx=4; group=1; election-tick=50"

This ensures the Raft leader stays in the segment closest to application servers, avoiding unnecessary write latency degradation from suboptimal leader placement after elections.

Default behavior is unchanged — without specifying election-tick, nodes use the previous value of 20 (2 second timeout).

Checklist

• The PR title follows the
  Conventional Commits syntax, leading
  with fix:, feat:, chore:, ci:, etc.
• Code compiles correctly and linting (via trunk) passes locally
• Tests added for new functionality, or regression tests for bug fixes added as applicable
• For public APIs, new features, etc., a PR on the
  docs repo staged and linked here.