Advance coordinator implementation

- Common
	- Move ErrTODO and ErrDone to common for usage where needed.
- Coordinator
	- Move prover types to prover package
	- Handle reorgs, stopping the pipeline when necessary
	- Handle ethereum transaction errors by stopping the pipeline
	- In case of ethereum transaction revert, check for known revert causes
	  (more revert causes can be added to handle more cases)
	- Fix skipped transactions in TxManager confirmation logic
	- Cancel and wait for provers to be ready
	- Connect L2DB to:
		- purge l2txs due to timeout
		- mark l2txs at the different states
	- Connect HistoryDB to query L1UserTxs to forge in an L1Batch
- L2DB
	- Skip update functions when the input slices have no values (to avoid a
	  query with no values that results in an SQL error)
- StateDB
	- In LocalStateDB, fix Reset when mt == nil
- Prover (new package)
	- Rename the interface to Prover
	- Rename the mock struct to Mock
	- Extend Prover interface methods to provide everything required by the
	  coordinator
	- Begin implementing required http client code to interact with server
	  proof (not tested)
- Synchronizer:
	- Add LastForgeL1TxsNum to Stats
- Test/Client
	- Update Auction logic to track slots in which there's no forge during
	  the time before the deadline (following the solidity implementation)

coordinator/proverspool.go

@@ -0,0 +1,38 @@
+package coordinator
+
+import (
+	"context"
+
+	"github.com/hermeznetwork/hermez-node/common"
+	"github.com/hermeznetwork/hermez-node/log"
+	"github.com/hermeznetwork/hermez-node/prover"
+	"github.com/hermeznetwork/tracerr"
+)
+
+// ProversPool contains the multiple prover clients
+type ProversPool struct {
+	pool chan prover.Client
+}
+
+// NewProversPool creates a new pool of provers.
+func NewProversPool(maxServerProofs int) *ProversPool {
+	return &ProversPool{
+		pool: make(chan prover.Client, maxServerProofs),
+	}
+}
+
+// Add a prover to the pool
+func (p *ProversPool) Add(serverProof prover.Client) {
+	p.pool <- serverProof
+}
+
+// Get returns the next available prover
+func (p *ProversPool) Get(ctx context.Context) (prover.Client, error) {
+	select {
+	case <-ctx.Done():
+		log.Info("ServerProofPool.Get done")
+		return nil, tracerr.Wrap(common.ErrDone)
+	case serverProof := <-p.pool:
+		return serverProof, nil
+	}
+}