The original part of the version is kept as is, and Percona specific
information is added after.

For example, psql displays the following after this commit:

SELECT version();
PostgreSQL 17.6 (PostgreSQL) - Percona Server for PostgreSQL 17.6.1 on x86_64-pc-linux-gnu, compiled by gcc (Ubuntu 11.4.0-1ubuntu1~22.04) 11.4.0, 64-bit

Where the addition is the extra version at the end:

" - Percona Server for PostgreSQL 17.4.1"

with the ".1" being the Percona version number.

Similarly, commands show the same information:

bin/pg_ctl --version
pg_ctl (PostgreSQL) 17.6 - Percona Server for PostgreSQL 17.6.1

There are various reasons why one would want to create their own
implementation of a storage manager, among which are block-level compression,
encryption and offloading to cold storage. This patch is a first patch that
allows extensions to register their own SMgr.

Note, however, that this SMgr is not yet used - only the first SMgr to register
is used, and this is currently the md.c smgr. Future commits will include
facilities to select an SMgr for each tablespace.

With this change, mdcreate receives the old relfilelocator along
with the new for operations that create a new file for an existing
relation.

This is required for tde_heap in pg_tde.

…(#156)

and allow extensions to override it
For now, it extends on `pread` and `pwrite` from/into segment files.
This is the minimum we need for full XLog encryption with pg_de.

This commit modifies the build scripts so that `llvmjit.so` links
LLVM statically instead of dynamically. No other binary affected,
the change shouldn't be user visible at all.

The memory footprint of postgres is also unaffected: only the
`llvmjit.so` dynamic library is affected. If postgres doesn't use
JIT, the LLVM library isn't loaded, as before. If it uses JIT, it
is loaded as part of `llvmjit.so`, sa before.

The only change is the binary size of `llvmjit.so`.

Instead of checking for the oid of the AM we check for if the
access method of the relation uses the routine struct as the
heap does. This allows the tools to be support all access
methods which are simple copies of heap.

I am uncertain if this is something which the project would be
interested in but it decreases the size of our diff against
upstream.

Need to modify the existing license to include the portions of code unique to Percona

The pg_tde extension's custom storage manager needs to add its own AIO
callback to decrypt pages before the buffer manager's callback is called
but since the set of AIO callbacks is fixed we need to expose this API
so that extensions can register their own types of callbacks. This new
function for registering a callback is supposed to be called when
intializing shared memory.

We try to keep the number of callbacks low, setting it to 15, so that we
have the 4 built-in callbacks and 11 slots that extensions can use.

This commit reintroduces the old method signatures of some methods we
changed to support pg_tde. The changed method signatures still exists
with a "2" suffix, and all internal code calls the "2" methods.

The reintroduced original methods also check a global flag which can
disable them - if this happens, they report a FATAL error instead. This
variable is activated by pg_tde, which means that if something tries to
use these while pg_tde is loaded, and with that, possibly ignores its
file tracking mechanism, it reports an error.

In practice this means that timescaledb, or the columnar storage in
citus won't work if pg_tde is loaded in the shared_preload_libraries,
but they can be used with our distribution without pg_tde enabled.

Also verified that both citus and timescaledb do compile with these
changes, and their basic test suite can be run againts our fork.