Multiple blocks per file

Change the block storage layer again, this time with multiple files
per block, but tracked by txindex.dat database entries. The file
format is exactly the same as the earlier blk00001.dat, but with
smaller files (128 MiB for now).

The database entries track how many bytes each block file already
uses, how many blocks are in it, which range of heights is present
and which range of dates.
Pre-allocate block and undo files in chunks

Introduce a AllocateFileRange() function in util, which wipes or
at least allocates a given range of a file. It can be overriden
by more efficient OS-dependent versions if necessary.

Block and undo files are now allocated in chunks of 16 and 1 MiB,
respectively.
Ultraprune

This switches bitcoin's transaction/block verification logic to use a
"coin database", which contains all unredeemed transaction output scripts,
amounts and heights.

The name ultraprune comes from the fact that instead of a full transaction
index, we only (need to) keep an index with unspent outputs. For now, the
blocks themselves are kept as usual, although they are only necessary for
serving, rescanning and reorganizing.

The basic datastructures are CCoins (representing the coins of a single
transaction), and CCoinsView (representing a state of the coins database).
There are several implementations for CCoinsView. A dummy, one backed by
the coins database (coins.dat), one backed by the memory pool, and one
that adds a cache on top of it. FetchInputs, ConnectInputs, ConnectBlock,
DisconnectBlock, ... now operate on a generic CCoinsView.

The block switching logic now builds a single cached CCoinsView with
changes to be committed to the database before any changes are made.
This means no uncommitted changes are ever read from the database, and
should ease the transition to another database layer which does not
support transactions (but does support atomic writes), like LevelDB.

For the getrawtransaction() RPC call, access to a txid-to-disk index
would be preferable. As this index is not necessary or even useful
for any other part of the implementation, it is not provided. Instead,
getrawtransaction() uses the coin database to find the block height,
and then scans that block to find the requested transaction. This is
slow, but should suffice for debug purposes.
Batch block connection during IBD

During the initial block download (or -loadblock), delay connection
of new blocks a bit, and perform them in a single action. This reduces
the load on the database engine, as subsequent blocks often update an
earlier block's transaction already.
Transaction hash caching

Use CBlock's vMerkleTree to cache transaction hashes, and pass them
along as argument in more function calls. During initial block download,
this results in every transaction's hash to be only computed once.
Direct CCoins references

To prevent excessive copying of CCoins in and out of the CCoinsView
implementations, introduce a GetCoins() function in CCoinsViewCache
with returns a direct reference. The block validation and connection
logic is updated to require caching CCoinsViews, and exploits the
GetCoins() function heavily.
Automatically reorganize at startup to best known block

Given that the block tree database (chain.dat) and the active chain
database (coins.dat) are entirely separate now, it becomes legal to
swap one with another instance without affecting the other.

This commit introduces a check in the startup code that detects the
presence of a better chain in chain.dat that has not been activated
yet, and does so efficiently (in batch, while reusing the blk???.dat
files).
Prepare database format for multi-stage block processing

This commit adds a status field and a transaction counter to the block
indexes.
Use singleton block tree database instance
Flush and sync block data
LevelDB block and coin databases

Split off CBlockTreeDB and CCoinsViewDB into txdb-*.{cpp,h} files,
implemented by either LevelDB or BDB.

Based on code from earlier commits by Mike Hearn in his leveldb
branch.
Add LevelDB MemEnv support

Support LevelDB memory-backed environments, and use them in unit tests.
Add gettxout and gettxoutsetinfo RPCs
Remove BDB block database support
Merge pull request #1677 from sipa/ultraprune

Ultraprune: use a pruned-txout-set database for block validation
Merge pull request #1936 from sipa/morehashwriter

Use CHashWriter also in SignatureHash(), and for message signing
Merge pull request #1916 from jgarzik/caddrdb-bug

Fix: CAddrMan: verify pchMessageStart file marker, before reading address data
Update version numbers to 0.7.99
Merge pull request #1938 from sipa/v0799

Update version numbers to 0.7.99
Bugfix: do not mark all future coins spent