Drop discovery module

[hanahmily]

As per the discussion last month, the core contributors have decided to remove the "discovery" module that serves as the discovery service function. Instead, the "metadata" module will now take on this role by providing the cluster's topology of nodes and shards.

@lujiajing1126 This change concentrates on the interface of "metadata", I need the added operations to support other feats. The internal implementation is left to you as we discussed before.

• Update the CHANGES log.

[codecov-commenter]

Codecov Report

Merging #311 (c735498) into main (397d6d8) will decrease coverage by 0.18%.
The diff coverage is 37.08%.

@@            Coverage Diff             @@
##             main     #311      +/-   ##
==========================================
- Coverage   39.82%   39.64%   -0.18%     
==========================================
  Files         101      104       +3     
  Lines       10894    11101     +207     
==========================================
+ Hits         4338     4401      +63     
- Misses       6131     6266     +135     
- Partials      425      434       +9     

Files Changed	Coverage Δ
banyand/measure/measure.go	80.00% <ø> (-0.89%)	⬇️
banyand/measure/metadata.go	49.19% <ø> (-0.61%)	⬇️
banyand/metadata/schema/checker.go	33.33% <0.00%> (-8.49%)	⬇️
banyand/metadata/schema/node.go	0.00% <0.00%> (ø)
banyand/metadata/schema/schema.go	36.47% <0.00%> (-11.97%)	⬇️
banyand/metadata/schema/shard.go	0.00% <0.00%> (ø)
banyand/stream/metadata.go	53.24% <ø> (-0.90%)	⬇️
banyand/stream/stream.go	76.47% <ø> (-1.91%)	⬇️
banyand/liaison/grpc/discovery.go	37.98% <41.21%> (+27.67%)	⬆️
banyand/metadata/client.go	35.83% <50.00%> (+1.62%)	⬆️
... and 4 more

... and 1 file with indirect coverage changes

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