Former-commit-id: e8fe5b96b617c98f07abf8d0b34c889016778b57

Closes #738

This is, basically, the patch developed also for Isracoin with only
minor modifications.  Adaption and testing still needs to be done.

Fix typo in Read() error message

serialization now has no dependencies.

- ensures a consistent usage in header files
- also add a blank line after the copyright header where missing
- also remove orphan new-lines at the end of some files

At some places where it is possible (e. g., CheckInputs), use a const
version of CCoinsView instead of a non-const one.

Mark the "Get"/"Have" routines in CCoinsView and subclasses as "const".

Goal:  Gain live insight into the mempool.  Groundwork for future work
that caps mempool size.

Accepts the transaction into mined blocks at a higher (or lower) priority

New RPC methods: return an estimate of the fee (or priority) a
transaction needs to be likely to confirm in a given number of
blocks.

Mike Hearn created the first version of this method for estimating fees.
It works as follows:

For transactions that took 1 to N (I picked N=25) blocks to confirm,
keep N buckets with at most 100 entries in each recording the
fees-per-kilobyte paid by those transactions.

(separate buckets are kept for transactions that confirmed because
they are high-priority)

The buckets are filled as blocks are found, and are saved/restored
in a new fee_estiamtes.dat file in the data directory.

A few variations on Mike's initial scheme:

To estimate the fee needed for a transaction to confirm in X buckets,
all of the samples in all of the buckets are used and a median of
all of the data is used to make the estimate. For example, imagine
25 buckets each containing the full 100 entries. Those 2,500 samples
are sorted, and the estimate of the fee needed to confirm in the very
next block is the 50'th-highest-fee-entry in that sorted list; the
estimate of the fee needed to confirm in the next two blocks is the
150'th-highest-fee-entry, etc.

That algorithm has the nice property that estimates of how much fee
you need to pay to get confirmed in block N will always be greater
than or equal to the estimate for block N+1. It would clearly be wrong
to say "pay 11 uBTC and you'll get confirmed in 3 blocks, but pay
12 uBTC and it will take LONGER".

A single block will not contribute more than 10 entries to any one
bucket, so a single miner and a large block cannot overwhelm
the estimates.

Use the spent outpoint multimap to figure out which wallet transaction
outputs are unspent, instead of a vfSpent array that is saved
to disk.

Also changes mempool to store CTxMemPoolEntries
to keep track of when they enter/exit the pool.

Use misc methods of avoiding unnecesary header includes.
Replace int typedefs with int##_t from stdint.h.
Replace PRI64[xdu] with PRI[xdu]64 from inttypes.h.
Normalize QT_VERSION ifs where possible.
Resolve some indirect dependencies as direct ones.
Remove extern declarations from .cpp files.