feat: unify PEX operation

[tzdybal]

As discussed in #1882., #2221 (and during calls), PEX operation should be unified.

This PR proposes following changes:

• ignore seed mode flag -> all nodes should act the same
• all nodes should disconnect from PEX right after address book exchange - thank to reactor-specific peers, this will create open "slots" for PEX connections allowing new peers to build address book quickly

Missing features in this draft:

• warning log when seed mode is on (similar to like: https://github.com/celestiaorg/celestia-core/pull/2181/files#diff-375d57e386f20eaa5f09f02bb9d28bfc48ac3dca18d0325f59492208219e5618R614-R616, but more backward compatible)
• fully ignoring seed mode flag, especially when starting PEX reactor

  celestia-core/p2p/pex/pex_reactor.go

  Lines 163 to 169 in 96021b9

  	// Check if this node should run
  	// in seed/crawler mode
  	if r.config.SeedMode {
  	go r.crawlPeersRoutine()
  	} else {
  	go r.ensurePeersRoutine()
  	}

• ensure that there is always an open slot for PEX request

Resolves #1882.