[blk storage] Adopt the Orderer optimizations for the block storage #167

The Fabric-X Orderer carries its own copy of this package at
common/ledger/blkstorage.
It is a sibling fork, not a descendant — both trees come from Hyperledger Fabric's blkstorage, and
each has diverged since. cache.go, block_stream.go, config.go, metrics.go and
protobuf_util.go are still byte-identical; the rest differs in both directions.

The divergence that matters here has one cause. All three orderer ledgers — assembler, batcher, and
consenter — configure their store with AttrsToIndex: []IndexableAttr{IndexableAttrBlockNum} and
nothing else, so no transaction ID index exists anywhere in the orderer. The same is true of a
Fabric-X Committer sidecar with the transaction ID index disabled
(hyperledger/fabric-x-committer#784). Both therefore pay for per-transaction index information that
nothing ever reads. The orderer's fork stopped paying; this repo's copy still does.

This umbrella tracks porting those wins back. Three, in descending order of measured impact:

1. [blkstorage] Do not build tx index information no index will read #165 — do not build transaction index information no index will read, on the write path
   (serializeBlock). Addressed by [blkstorage] Do not build tx index information no index will read #166, which is a superset of the orderer's version: it derives the
   decision from the index configuration rather than a caller-passed bool, also skips the offsets and
   not just the transaction IDs, and pre-sizes the serialization buffer. Nothing left to port.
2. [blkstorage] Do not compute transaction IDs when deserializing a block #182 — the read path — extractData computes a transaction ID per envelope on every block read and
   deserializeBlock discards all of it. Not touched by [blkstorage] Do not build tx index information no index will read #166. This is the same unmarshal as [blkstorage] Do not build tx index information no index will read #165, on
   the other side of the store, and unlike [blkstorage] Do not build tx index information no index will read #165 it is wasted unconditionally rather than only for
   stores without a transaction ID index.
3. [blkstorage] Extract only the header when only the header is read #183 — header-only reads — retrieveBlockHeaderByNumber and retrieveFirstBlockNumFromFile parse
   every envelope and all metadata to read a three-field header.

Not in scope, and not to be adopted: the orderer's fork dropped snapshot bootstrap and export,
reset and rollback, and left TODOs where the snapshot front-boundary guards used to sit,
marking them for ledger pruning. This repo is also ahead of the orderer on AddBlockNoSync/Flush
with a sync before file rollover (#69), the context-aware blocksItr (#88), the ErrNotFound
sentinel (#90), and cockroachdb/errors. Each item below is therefore additive to this tree — there
is no orderer behaviour to take wholesale.