add LOCK() for proxy related data-structures

- fix #1560 by properly locking proxy related data-structures
- update GetProxy() and introduce GetNameProxy() to be able to use a
  thread-safe local copy from proxyInfo and nameproxyInfo
- update usage of GetProxy() all over the source to match the new
  behaviour, as it now fills a full proxyType object
- rename GetNameProxy() into HaveNameProxy() to be more clear
Merge pull request #1859 from Diapolo/proxy_locks

add LOCK() for proxy related data-structures
Merge branch 'canonical'
Merge branch 'fdatasync'
Merge branch 'threadimport'
Do not forward 0-writes to SHA256
Import LevelDB 1.5, it will be used for the transaction database.
Leveldb Windows port by Edouard Alligand, adapted for MingW by me.
Disable libsnappy detection in LevelDB
Do not force -std=c++0x for MingW builds
Makefile integration of LevelDB
Compact serialization for variable-length integers

Variable-length integers: bytes are a MSB base-128 encoding of the number.
The high bit in each byte signifies whether another digit follows. To make
the encoding is one-to-one, one is subtracted from all but the last digit.
Thus, the byte sequence a[] with length len, where all but the last byte
has bit 128 set, encodes the number:

  (a[len-1] & 0x7F) + sum(i=1..len-1, 128^i*((a[len-i-1] & 0x7F)+1))

Properties:
* Very small (0-127: 1 byte, 128-16511: 2 bytes, 16512-2113663: 3 bytes)
* Every integer has exactly one encoding
* Encoding does not depend on size of original integer type
Compact serialization for variable-length integers: tests
Add CCoins: pruned list of transaction outputs

The CCoins class represents a pruned set of transaction outputs from
a given transaction. It only retains information about its height in
the block chain, whether it was a coinbase transaction, and its
unspent outputs (script + amount).

It has a custom serializer that has very low redundancy.
Compact serialization for scripts

Special serializers for script which detect common cases and encode
them much more efficiently. 3 special cases are defined:
* Pay to pubkey hash (encoded as 21 bytes)
* Pay to script hash (encoded as 21 bytes)
* Pay to pubkey starting with 0x02, 0x03 or 0x04 (encoded as 33 bytes)

Other scripts up to 121 bytes require 1 byte + script length. Above
that, scripts up to 16505 bytes require 2 bytes + script length.
Compact serialization for amounts

Special serializer/deserializer for amount values. It is optimized for
values which have few non-zero digits in decimal representation. Most
amounts currently in the txout set take only 1 or 2 bytes to
represent.
Compact serialization for amounts: tests

Test cases to verify the correctness of the custom amount serializer.
Add CTxUndo: transaction undo information

The CTxUndo class encapsulates data necessary to undo the effects of
a transaction on the txout set, namely the previous outputs consumed
by it (script + amount).
One file per block

Refactor of the block storage code, which now stores one file per block.
This will allow easier pruning, as blocks can be removed individually.
Preliminary undo file creation

Create files (one per block) with undo information for the transactions
in it.